Adrenergic Agonists: Classification, Pharmacology and Clinical Use

Adrenergic Agonists (Sympathomimetics): Comprehensive Pharmacological Guide, Dosing, and Safe Clinical Use

What this comprehensive chapter covers This guide provides an extensive, medicine-by-medicine deep dive into adrenergic pharmacology. Synthesizing standard physiological foundations with clinical applications, it ensures students and practitioners understand the biosynthesis, mechanism of action (MOA), pharmacokinetics, receptor profiles, and precise indications for each major adrenergic agonist. Detailed sections are provided for every drug, segregating common side effects from severe adverse events, outlining strict contraindications, and cataloging vital drug-drug interactions.

Critical Pre-Prescription Safety Review

Identify the exact clinical problem: Anaphylaxis, acute asthma exacerbation, cardiogenic shock, septic shock, symptomatic bradycardia, and orthostatic hypotension possess entirely different pathological mechanisms requiring highly specific receptor targeting.
Baseline & Continuous Assessment: Check BP, pulse rate, cardiac rhythm, continuous ECG (for IV infusions), oxygenation (SpO₂), serum K⁺, blood glucose, renal function, hydration/volume status, and pregnancy status.
Verify Formulations & Concentrations: A classic lethal error is confusing adrenaline concentrations. Adrenaline 1 mg/mL (1:1000) is for IM use only. Adrenaline 0.1 mg/mL (1:10,000) is for IV resuscitation. They are never interchangeable [1].
IV Vasoactive Protocol: Always use a central line when possible, an infusion pump, a traceable protocol, and frequent reassessment with documented targets (e.g., MAP > 65 mmHg, improved capillary refill, urine output > 0.5 mL/kg/hr).

Introduction to Adrenergic Pharmacology

The sympathetic nervous system (SNS) regulates the body’s “fight or flight” response through the release of endogenous catecholamines (dopamine, noradrenaline, and adrenaline). Understanding how to manipulate this system pharmacologically requires a firm grasp of catecholamine life cycles and receptor mechanics.

Biosynthesis: Synthesized in adrenergic neurons and the adrenal medulla. The pathway begins with the amino acid Tyrosine → DOPA (via Tyrosine Hydroxylase, the rate-limiting step) → Dopamine → Noradrenaline (Norepinephrine) → Adrenaline (Epinephrine) [1].
Storage & Release: Catecholamines are stored in presynaptic vesicles (protected from degradation) and released into the synaptic cleft upon calcium influx triggered by an action potential.
Termination of Action: Reuptake into the presynaptic neuron (via NET/DAT) is the primary termination method. Enzymatic degradation is handled by Monoamine Oxidase (MAO) in the mitochondria and Catechol-O-Methyltransferase (COMT) in the synaptic space and liver. The final urinary metabolite for epinephrine and norepinephrine is Vanillylmandelic Acid (VMA) [1].

1. Receptor Foundation & Second Messengers

Receptor	G-Protein / 2nd Messenger	Major Physiologic Effect	Clinical Consequence & Application
α₁	G_q → PLC → IP₃/DAG → ↑ Intracellular Ca²⁺	Smooth muscle contraction (arterial/venous vasoconstriction), mydriasis (pupil dilation), contraction of bladder sphincter.	Raises Systemic Vascular Resistance (SVR) and BP; relieves nasal mucosal edema; causes urinary retention and potential tissue ischemia.
α₂	G_i → inhibits Adenylyl Cyclase → ↓ cAMP	Presynaptic inhibition of noradrenaline release; decreases central sympathetic outflow.	Lowers BP and heart rate centrally; abrupt withdrawal causes severe rebound hypertension.
β₁	G_s → stimulates Adenylyl Cyclase → ↑ cAMP	Increased chronotropy (heart rate), inotropy (contractility), dromotropy (AV conduction), and renin release (kidneys).	Raises cardiac output; increases myocardial oxygen demand; may provoke angina or tachyarrhythmias.
β₂	G_s → stimulates Adenylyl Cyclase → ↑ cAMP	Smooth muscle relaxation (bronchodilation, uterine relaxation, vasodilation of skeletal muscle beds), glycogenolysis, cellular K⁺ uptake.	Relieves bronchospasm; suppresses premature labor; triggers fine muscle tremor, reflex tachycardia, hyperglycemia, and hypokalemia.
D₁	G_s → stimulates Adenylyl Cyclase → ↑ cAMP	Vasodilation in renal, mesenteric, and coronary beds.	Improves renal and splanchnic blood flow at low dopamine doses (though “renal-dose dopamine” to prevent AKI is no longer supported by evidence).

Detailed Drug Profiles

2. Adrenaline (Epinephrine)

Class & Receptor Profile: Endogenous catecholamine; potent direct non-selective agonist at α₁, α₂, β₁, and β₂ receptors.
Pharmacokinetics: Poorly absorbed orally (rapidly destroyed by gut MAO/COMT). Given IM, SC, IV, or via inhalation. Does not readily cross the blood-brain barrier. Extremely short half-life (~2 minutes).
Mechanism of Action (MOA): Dose-dependent effects. Low doses primarily stimulate β receptors (causing vasodilation and increased heart rate). High doses heavily stimulate α₁ receptors (causing profound vasoconstriction that overrides β₂ vasodilation). β₂ agonism provides strong bronchodilation and inhibits mast cell degranulation.
Clinical Indications: Acute anaphylaxis (first-line), cardiac arrest (VF, pulseless VT, asystole, PEA), severe acute asthma (when inhaled agents fail), combined with local anesthetics (to induce local vasoconstriction, reducing systemic absorption and prolonging anesthesia).
Dosage & Administration:
- Anaphylaxis: 0.3–0.5 mg IM of 1 mg/mL (1:1000) into the anterolateral mid-thigh. Repeat every 5–15 minutes PRN.
- Cardiac Arrest: 1 mg IV/IO of 0.1 mg/mL (1:10,000) every 3–5 minutes during CPR.
Contraindications: No absolute contraindications in life-threatening anaphylaxis. Relative contraindications in non-emergencies: narrow-angle glaucoma, severe cardiac disease, non-anaphylactic shock, and known sulfite allergy (present in some formulations) [2].
Common Side Effects: Tremor, restlessness, anxiety, tension, palpitations, headache, sweating, pallor, and nausea.
Severe Adverse Effects: Ventricular arrhythmias (VF/VT), fatal myocardial ischemia (due to massive increase in O₂ demand), severe hypertension, cerebrovascular accident (hemorrhage), and pulmonary edema. Extravasation can cause severe tissue necrosis.
Drug Interactions:
- Non-selective Beta-Blockers (e.g., Propranolol): Blocks β₂ vasodilation, leaving α₁ vasoconstriction unopposed → life-threatening hypertensive crisis with reflex bradycardia.
- MAOIs & TCAs: Prevent catecholamine breakdown/reuptake → exaggerated pressor effects.
- Halogenated Anesthetics (e.g., Halothane): Sensitize the myocardium to epinephrine, greatly increasing the risk of arrhythmias.
Monitoring: Continuous ECG, BP, heart rate, SpO₂, blood glucose (can cause hyperglycemia), and serum potassium (can cause transient hypokalemia).

3. Noradrenaline (Norepinephrine)

Class & Receptor Profile: Endogenous catecholamine; potent α₁ and α₂ agonist, moderate β₁ agonist, minimal to no β₂ activity.
Pharmacokinetics: Administered exclusively via IV infusion. Rapid onset (1-2 mins), very short duration (metabolized by COMT/MAO).
Mechanism of Action (MOA): Massive α₁ stimulation leads to intense peripheral vasoconstriction, significantly increasing systemic vascular resistance (SVR) and both systolic and diastolic blood pressures. While it has β₁ chronotropic effects, the sudden surge in blood pressure often triggers a vagal baroreceptor reflex that causes reflex bradycardia, masking the direct heart rate increase.
Clinical Indications: First-line vasopressor for distributive/septic shock, neurogenic shock, and profound hypotension unresolved by adequate fluid resuscitation.
Dosage & Administration: Titrated IV infusion via a central venous catheter (to avoid extravasation). Typically starts at 8–12 mcg/min and titrated to achieve a Mean Arterial Pressure (MAP) > 65 mmHg.
Contraindications: Uncorrected hypovolemia (must replace fluids first), mesenteric or peripheral vascular thrombosis, and severe hypoxia/hypercarbia.
Common Side Effects: Anxiety, headache, reflex bradycardia, photophobia.
Severe Adverse Effects: Peripheral, mesenteric, or renal ischemia (can lead to digit necrosis or bowel infarction), severe hypertension, and fatal arrhythmias. Extravasation Toxicity: Causes intense localized vasoconstriction and sloughing/necrosis. (Antidote: SC infiltration of Phentolamine, an α-blocker).
Drug Interactions: MAOIs, TCAs, and SNRIs amplify cardiovascular effects. Concurrent use with oxytocin or ergot alkaloids can cause catastrophic hypertension.
Monitoring: Continuous intra-arterial BP monitoring (A-line), MAP, peripheral perfusion (capillary refill, extremity temperature), lactate clearance, and urine output (to ensure renal perfusion).

4. Dopamine

Class & Receptor Profile: Endogenous catecholamine (precursor to noradrenaline). Dose-dependent effects on D₁, β₁, and α₁ receptors.
Pharmacokinetics: IV administration only. Half-life is roughly 2 minutes. Metabolized by MAO and COMT.
Mechanism of Action (MOA):
- Low dose (1–5 mcg/kg/min): D₁ receptor activation → vasodilation in renal and mesenteric beds.
- Moderate dose (5–15 mcg/kg/min): β₁ activation → increases cardiac contractility and heart rate (positive inotropy and chronotropy).
- High dose (>15 mcg/kg/min): α₁ activation → generalized vasoconstriction overriding dopaminergic vasodilation.
Clinical Indications: Severe symptomatic bradycardia, cardiogenic shock, and hypotensive states (second-line to noradrenaline due to higher arrhythmogenic potential).
Dosage & Administration: IV infusion titrated strictly via pump (typically 2–20 mcg/kg/min).
Contraindications: Pheochromocytoma, uncorrected tachyarrhythmias, or Ventricular Fibrillation.
Common Side Effects: Nausea, vomiting, headache, palpitations.
Severe Adverse Effects: Tachyarrhythmias (higher risk than noradrenaline), angina/myocardial ischemia, wide QRS complex, and tissue necrosis upon extravasation.
Drug Interactions: MAOIs severely impair dopamine breakdown (dose must be reduced to 1/10th of normal if MAOI is present). Phenytoin given with dopamine may cause severe hypotension and bradycardia.
Monitoring: Continuous ECG, BP, urine output, peripheral perfusion, and central line site.

5. Dobutamine

Class & Receptor Profile: Synthetic catecholamine; primarily a β₁ agonist, with minor β₂ and α₁ effects. (Administered as a racemic mixture where (+) isomer is a potent β₁/β₂ agonist and α₁ antagonist, while (-) isomer is an α₁ agonist. The alpha effects largely cancel out, leaving prominent β₁ inotropy and mild β₂ vasodilation).
Mechanism of Action (MOA): Directly increases myocardial contractility and stroke volume without a massive increase in heart rate. The mild β₂ activation can lead to a slight decrease in systemic vascular resistance (afterload reduction).
Clinical Indications: Acute decompensated heart failure with low cardiac output, cardiogenic shock (often combined with a vasopressor like noradrenaline), and pharmacological stress echocardiography.
Dosage & Administration: IV infusion (2–20 mcg/kg/min). Titrated to cardiac index and perfusion markers.
Contraindications: Idiopathic Hypertrophic Subaortic Stenosis (IHSS) or Hypertrophic Obstructive Cardiomyopathy (HOCM)—increased contractility worsens the outflow tract obstruction.
Common Side Effects: Tachycardia, mild hypotension initially (due to β₂ vasodilation before cardiac output increases), headache, nausea.
Severe Adverse Effects: Worsening of ventricular arrhythmias, rapid ventricular response in patients with Atrial Fibrillation, myocardial ischemia, and extreme hypertension or hypotension.
Drug Interactions: Beta-blockers directly antagonize its efficacy. Co-administration with nitroprusside can synergistically improve cardiac output but raises the risk of severe hypotension.
Monitoring: Continuous ECG, BP, echocardiographic parameters, central venous pressure (CVP), and pulmonary capillary wedge pressure (PCWP).

6. Isoprenaline (Isoproterenol)

Class & Receptor Profile: Synthetic catecholamine; potent, non-selective pure β agonist (equal affinity for β₁ and β₂), with zero α activity.
Mechanism of Action (MOA): Intense β₁ stimulation maximally increases heart rate and contractility. Intense β₂ stimulation causes massive vasodilation in skeletal muscle vascular beds, causing a sharp drop in Systemic Vascular Resistance (SVR) and diastolic blood pressure.
Clinical Indications: Refractory symptomatic bradycardia or high-degree AV block when a pacemaker is unavailable; historically used for asthma but replaced by selective agents.
Dosage & Administration: Titrated IV infusion only. Highly protocol-specific.
Contraindications: Angina, acute myocardial infarction, tachyarrhythmias, and digitalis toxicity.
Common Side Effects: Flushing, palpitations, tremor, headache.
Severe Adverse Effects: Cardiac Ischemia—this drug is notorious for causing ischemia because it drastically increases cardiac oxygen demand (via β₁) while simultaneously decreasing coronary artery perfusion pressure (due to the β₂-mediated drop in diastolic BP). High risk of fatal ventricular arrhythmias.
Drug Interactions: Epinephrine/Dobutamine (additive cardiotoxicity).
Monitoring: Continuous ECG, arterial line BP (watch for collapsing diastolic pressures), troponins, and ischemic signs on ECG.

7. Phenylephrine

Class & Receptor Profile: Direct-acting, synthetic selective α₁ agonist.
Mechanism of Action (MOA): Stimulates α₁ receptors to cause direct veno- and arterial constriction. Lacks β-receptor activity, so it does not directly stimulate the heart. The resulting spike in blood pressure consistently triggers a vagal reflex, leading to reflex bradycardia. It also directly contracts the radial muscle of the iris (pupil dilation) without affecting accommodation.
Clinical Indications: IV vasopressor for anesthesia-induced hypotension (e.g., during spinal anesthesia), topical nasal decongestant, ophthalmic mydriatic for retinal exams, and treatment of SVT (using reflex bradycardia to terminate the rhythm).
Dosage & Administration: IV bolus (e.g., 50–100 mcg) or infusion. Oral formulations (though largely proven ineffective compared to placebo) and nasal sprays are available.
Contraindications: Severe hypertension, preexisting bradycardia, narrow-angle glaucoma (ophthalmic form), and severe peripheral vascular disease.
Common Side Effects: Piloerection (goosebumps), headache, restlessness, sinus bradycardia.
Severe Adverse Effects: Rebound nasal congestion (if topical spray used > 3-5 days), tissue necrosis from IV extravasation, severe hypertensive crisis, and decreased cardiac output (due to increased afterload combined with reflex bradycardia).
Drug Interactions: MAOIs and TCAs dramatically potentiate hypertensive effects. Concurrent use with oxytocics causes severe continuous hypertension.
Monitoring: BP, HR, peripheral perfusion.

8. Midodrine

Class & Receptor Profile: Oral prodrug; active metabolite (desglymidodrine) is a selective α₁ agonist.
Mechanism of Action (MOA): Increases arteriolar and venous tone, augmenting systemic vascular resistance and preventing blood from pooling in the lower extremities when standing.
Clinical Indications: Severe, symptomatic orthostatic hypotension that severely impairs quality of life (after lifestyle measures fail).
Dosage & Administration: 2.5–10 mg orally three times daily. Critical timing: Must be taken during waking hours (e.g., 8 AM, 12 PM, 4 PM). Do not take within 4 hours of bedtime.
Contraindications: Severe organic heart disease, acute renal disease, urinary retention, thyrotoxicosis, and persistent supine hypertension.
Common Side Effects: Piloerection, pruritus (scalp tingling/itching is very common), shivering, and urinary urgency/retention (due to internal sphincter contraction).
Severe Adverse Effects: Supine hypertension (can cause stroke).
Drug Interactions: Alpha-blockers (e.g., prazosin, tamsulosin) completely negate its effects. Digoxin can worsen drug-induced bradycardia.
Monitoring: Regular measurement of orthostatic vitals: supine, sitting, and standing blood pressures. Renal function.

9. Salbutamol (Albuterol)

Class & Receptor Profile: Short-Acting Beta-2 Agonist (SABA).
Mechanism of Action (MOA): Binds to β₂ receptors in airway smooth muscle → activates Adenylyl Cyclase → increases intracellular cAMP → activates Protein Kinase A (PKA) → lowers intracellular calcium and inhibits myosin light-chain kinase, resulting in smooth muscle relaxation and bronchodilation. Also activates Na⁺/K⁺-ATPase, driving potassium into cells.
Clinical Indications: Acute asthma exacerbations, COPD bronchospasm, prevention of exercise-induced asthma, and as a temporizing adjunct for hyperkalemia.
Dosage & Administration:
- Inhaler (MDI): 90–100 mcg/puff; typically 2 puffs Q4-Q6H PRN.
- Nebulized: 2.5–5 mg diluted in saline for acute exacerbations.
Contraindications: Known hypersensitivity. Extreme caution in severe IHD, active tachyarrhythmias, and poorly controlled hyperthyroidism [3].
Common Side Effects: Fine skeletal muscle tremor (direct β₂ effect on muscle spindles), palpitations, sinus tachycardia, nervousness, headache.
Severe Adverse Effects: Paradoxical bronchospasm (can be fatal; discontinue immediately), profound hypokalemia (especially via nebulizer), serious arrhythmias, and high doses can cause lactic acidosis and worsened diabetic ketoacidosis [3].
Drug Interactions:
- Non-selective Beta-blockers (e.g., Carvedilol, Propranolol): Competitively antagonize bronchodilation and can precipitate severe bronchospasm.
- Non-potassium sparing diuretics (Loop/Thiazides): Can dangerously worsen hypokalemia and ECG changes [3].
- Digoxin: Albuterol can lower serum digoxin levels [3].
Monitoring: Peak Expiratory Flow Rate (PEFR), SpO₂, work of breathing, lung auscultation, serum potassium, and blood glucose.
Clinical Pearls: Using a SABA >2 times per week indicates uncontrolled asthma and necessitates stepping up anti-inflammatory controller therapy (e.g., Inhaled Corticosteroids). Salbutamol fixes the symptom (spasm), not the disease (inflammation). Ensure proper spacer technique.

10. Terbutaline

Class & Receptor Profile: Selective β₂ agonist.
Mechanism of Action (MOA): Similar to salbutamol; increases cAMP to cause bronchial smooth muscle relaxation. It also relaxes uterine smooth muscle.
Clinical Indications: Bronchospasm (can be given Subcutaneously in emergency settings when inhaled routes are impossible). Off-label: Historically used as a tocolytic to arrest premature labor.
Dosage & Administration: 0.25 mg SC for acute bronchospasm. Repeat in 15-30 mins if required (max 0.5 mg in 4 hours).
Contraindications: FDA Black Box Warning: Oral/prolonged injectable use (>48-72 hours) for preventing preterm labor is contraindicated due to severe maternal cardiotoxicity and death. Avoid in severe ischemic heart disease.
Common Side Effects: Tremor, maternal and fetal tachycardia, restlessness, flushing, hypokalemia, and hyperglycemia.
Severe Adverse Effects: Maternal pulmonary edema, myocardial ischemia, life-threatening arrhythmias, and severe lactic acidosis.
Drug Interactions: Additive hypokalemia with corticosteroids and diuretics. Beta-blockers antagonize its action.
Monitoring: When used in obstetrics (highly restricted): continuous fetal heart rate monitoring, maternal ECG, potassium, glucose, and fluid intake/output (risk of pulmonary edema).

11. Clonidine

Class & Receptor Profile: Central α₂ agonist.
Mechanism of Action (MOA): Stimulates presynaptic α₂ receptors in the vasomotor centers of the brainstem (medulla oblongata). This triggers a negative feedback loop that severely dampens central sympathetic outflow. Reduces peripheral noradrenaline release, subsequently lowering heart rate, blood pressure, and vascular resistance.
Clinical Indications: Hypertensive urgency/resistant hypertension, ADHD, management of opioid/alcohol/nicotine withdrawal (reduces autonomic hyperarousal), and as an analgesic adjunct in epidurals.
Dosage & Administration: Oral (commonly starting 50–100 mcg BID, titrated slowly) or transdermal patch (replaced weekly).
Contraindications: Severe bradyarrhythmias, heart block, and severe depressive disorders (can exacerbate depression). Caution in renal failure.
Common Side Effects: Severe dry mouth (xerostomia), drowsiness, sedation, constipation, impotence, and dizziness.
Severe Adverse Effects: Rebound Hypertensive Crisis: Abrupt cessation causes a massive surge in catecholamine release, leading to severe hypertension, tachycardia, tremors, and agitation. Must be tapered slowly over a week.
Drug Interactions:
- Beta-Blockers: If a patient is on both, the beta-blocker MUST be withdrawn several days before tapering clonidine, otherwise unopposed alpha-vasoconstriction during clonidine withdrawal will cause catastrophic hypertension.
- CNS depressants (alcohol, opioids, benzos) massively increase sedation.
Monitoring: Postural blood pressures, mental status/sedation levels, and strict adherence checks.

12. Ephedrine

Class & Receptor Profile: Mixed-acting sympathomimetic. Directly acts on α and β receptors AND indirectly causes the release of stored noradrenaline from nerve terminals.
Mechanism of Action (MOA): Highly lipid-soluble (crosses the blood-brain barrier). Causes peripheral vasoconstriction (α₁), increased heart rate/contractility (β₁), bronchodilation (β₂), and mild CNS stimulation (alertness/anxiety).
Clinical Indications: Primarily used by anesthesiologists to treat hypotension secondary to neuraxial (spinal/epidural) anesthesia. Historically used for asthma and nasal congestion.
Dosage & Administration: IV bolus (typically 5–10 mg incremental doses) under strict anesthetic monitoring.
Contraindications: Uncontrolled hypertension, hyperthyroidism, closed-angle glaucoma, and concurrent MAOI use.
Common Side Effects: Anxiety, insomnia, tremor, tachycardia, urinary retention.
Severe Adverse Effects: Arrhythmias, hypertensive crisis, cerebral hemorrhage.
Pharmacological Concept – Tachyphylaxis: Repeated dosing within a short period yields progressively smaller responses because the neuronal stores of noradrenaline become depleted.
Drug Interactions: MAOIs (causes hypertensive crisis—requires a 14-day washout). Halogenated anesthetics (arrhythmia risk).
Monitoring: Continuous BP, ECG, and awareness of cumulative dosing.

13. Pseudoephedrine & Topical Decongestants (Oxymetazoline / Xylometazoline)

Pseudoephedrine:
- Class & MOA: Stereoisomer of ephedrine; mixed α/β agonist. Causes vasoconstriction of the engorged, edematous nasal mucosal capillary beds via α₁ stimulation, widening the airway.
- Indications & Dosing: Symptomatic relief of nasal congestion. Typical oral dose: 60 mg Q4-Q6H (Max 240 mg/day). Often kept behind the pharmacy counter due to illicit diversion for methamphetamine synthesis.
- Contraindications: Severe hypertension, severe CAD, prostatic hypertrophy, and MAOI use.
- Adverse Effects: Insomnia, nervousness, palpitations, elevated BP, and urinary retention (especially in older men with BPH).
Topical Decongestants (Xylometazoline, Oxymetazoline):
- Class & MOA: Direct acting α₁ and partial α₂ agonists applied directly to the nasal mucosa. Fast onset vasoconstriction.
- Golden Rule: DO NOT use for more than 3 to 5 consecutive days.
- Severe Adverse Effect: Prolonged use leads to receptor downregulation and mucosal ischemia, resulting in severe rebound nasal congestion (Rhinitis Medicamentosa) that is incredibly difficult to treat.
- Counseling: Use strict hygiene (don’t share bottles), adhere to the 3-day rule, and discard safely away from small children (ingestion of even small amounts by toddlers can cause profound lethargy, bradycardia, and coma due to central α₂ stimulation).

14. Indirect Agonists: Amphetamine, Methylphenidate, and Tyramine

Amphetamine & Dextroamphetamine:
- MOA: Enters the presynaptic nerve terminal via the reuptake transporter (NET/DAT), enters the vesicles via VMAT, and displaces huge amounts of Noradrenaline and Dopamine into the cytoplasm. It then reverses the reuptake transporters, pumping catecholamines into the synapse.
- Indications: ADHD, Narcolepsy.
- Adverse Effects: Severe addiction liability, psychosis, profound insomnia, appetite suppression (weight loss/growth retardation in children), tachycardia, and hypertension.
- Monitoring: Pediatric growth charts, BP, HR, sleep hygiene, and diversion/misuse screening.
Methylphenidate:
- MOA: Binds to and blocks the dopamine (DAT) and noradrenaline (NET) reuptake transporters, causing neurotransmitter accumulation in the synaptic cleft (similar mechanism to cocaine, but with slower pharmacokinetics).
- Indications & Use: First-line for ADHD. Available in numerous formulations (immediate, sustained, osmotic-release). Never crush or chew sustained-release formulas.
- Adverse Effects & Monitoring: Similar to amphetamines. Monitor BP, pulse, sleep, appetite, and emergence of motor tics.
Tyramine (Dietary Indirect Agonist):
- Pharmacology: A naturally occurring trace amine found in aged cheeses, cured meats, fermented beverages (tap beer, red wine), and soy sauce. It acts as an indirect sympathomimetic by displacing stored noradrenaline.
- The “Cheese Reaction”: Normally, dietary tyramine is rapidly destroyed by MAO-A in the GI tract and liver. If a patient is taking an older, non-selective Monoamine Oxidase Inhibitor (MAOI, e.g., Phenelzine, Tranylcypromine), tyramine enters the systemic circulation untouched, displacing massive amounts of noradrenaline and triggering a potentially lethal hypertensive crisis (manifesting as occipital headache, stiff neck, tachycardia, nausea, and stroke) [1].

Knowledge Check & Final Review

Epinephrine: 1:1,000 is for IM thigh injection in anaphylaxis; 1:10,000 is for IV cardiac arrest. Concentration errors are lethal.
Norepinephrine: First-line for septic shock. Prioritize fluid replacement first. Watch for tissue necrosis.
Dobutamine: Select it for acute heart failure/cardiogenic shock to improve contractility, but be wary of increasing O₂ demand and triggering arrhythmias.
Salbutamol: Short-acting reliever. Frequent use means the asthma is out of control. Watch for tremor and hypokalemia.
Clonidine: Never stop abruptly. Taper to prevent life-threatening rebound hypertension.
Topical Nasal Decongestants: Maximum 3-5 days to avoid Rhinitis Medicamentosa.

Doctors Revision