Cholinergic Agonists: Muscarinic, Nicotinic and Anticholinesterase Drugs

Learning outcomes

• Differentiate direct muscarinic agonists, nicotine-receptor agonists and indirect AChE inhibitors.
• Predict organ effects using M₁, M₂, M₃, N_N and N_M receptors.
• Give drug-by-drug indications, doses, adverse effects, contraindications and interactions.
• Recognise cholinergic crisis and organophosphate poisoning as emergencies.

Bethanechol

Mechanism and receptor profile

• Synthetic choline ester that directly stimulates muscarinic receptors; it has negligible nicotinic action.
• Increases detrusor contraction and GI smooth-muscle tone; it is resistant to AChE relative to acetylcholine.

Formulation and common adult teaching dose

• Oral tablets; common teaching dose 10–50 mg orally three or four times daily, using the lowest effective dose.
• Give on an empty stomach if possible: nausea/vomiting may be more troublesome near meals. Check local product guidance.

Indications

• Acute postoperative/postpartum non-obstructive urinary retention.
• Selected neurogenic bladder atony with retention after mechanical obstruction has been excluded.

Common side effects

• Abdominal cramps and diarrhoea: M₃ stimulation increases gut tone/motility.
• Salivation, sweating and flushing: glandular and autonomic stimulation.
• Urinary urgency: detrusor stimulation may be excessive.
• Headache/dizziness: can accompany BP or autonomic changes.

Serious adverse effects

• Bronchospasm/bronchorrhoea: potentially severe in asthma or COPD.
• Bradycardia/hypotension: muscarinic cardiac and vascular effects can compromise perfusion.
• Bladder rupture risk: do not stimulate a bladder against an unrecognised obstruction.

Contraindications and cautions

• Contraindicated/cautioned in mechanical urinary or GI obstruction, peptic ulcer disease, asthma, marked bradycardia/hypotension, coronary disease, epilepsy and Parkinsonism.
• Confirm that retention is functional, not due to enlarged prostate, stone, clot, stricture or pelvic pathology.

Interactions

• Other cholinergic drugs and AChE inhibitors increase muscarinic toxicity.
• Antimuscarinics oppose the desired effect; beta blockers can worsen clinically important bradycardia.

Monitoring and counselling

• Assess voiding, post-void residual where available, abdominal pain, BP, pulse, wheeze and diarrhoea.
• Seek urgent review for severe colic, inability to pass urine, fainting, wheeze or profuse secretions.

Pilocarpine

Mechanism and receptor profile

• Direct muscarinic agonist that stimulates exocrine secretion and contracts the iris sphincter/ciliary muscle.
• In the eye it causes miosis and ciliary muscle contraction, increasing trabecular outflow; orally it increases salivation.

Formulations and common adult teaching doses

• Oral: commonly 5 mg three times daily for radiation-related xerostomia; Sjögren syndrome regimens may use 5 mg four times daily, subject to product guidance.
• Ophthalmic: concentration and frequency depend on glaucoma indication and ophthalmology protocol; do not copy an oral dose into eye-drop prescribing.

Indications

• Dry mouth from salivary-gland hypofunction after head/neck radiotherapy or in Sjögren syndrome.
• Selected glaucoma/ocular uses under ophthalmology care.

Common side effects

• Sweating: a dose-limiting consequence of glandular M₃ stimulation.
• Salivation, nausea, diarrhoea and urinary frequency: general muscarinic effects.
• Blurred vision/headache: miosis and accommodation spasm alter visual function, particularly in dim light.

Serious adverse effects

• Bronchospasm or increased bronchial secretions: dangerous in uncontrolled airway disease.
• Bradyarrhythmia/hypotension: risk is greater in important cardiovascular disease.
• Retinal detachment risk: rare ocular concern in susceptible patients; new flashes/floaters require urgent eye review.

Contraindications and cautions

• Avoid where miosis is undesirable, such as narrow-angle glaucoma unless specialist-directed; use caution in asthma, peptic ulcer and cardiovascular disease.
• Patients with night-driving duties need counselling about impaired dark adaptation/blurred vision.

Interactions

• Antimuscarinics reduce efficacy; other cholinergic medicines increase toxicity.
• BP/rate-lowering medicines may magnify symptomatic hypotension or bradycardia.

Monitoring and counselling

• Assess salivary response, hydration, sweating, wheeze, GI tolerance and ocular symptoms.
• For drops, teach exact instillation and punctal occlusion; avoid driving in poor light until vision effects are understood.

Cevimeline

Mechanism and receptor profile

• Direct muscarinic agonist with relative M₁/M₃ activity; promotes salivary secretion.
• It is a practical alternative to pilocarpine where available, but shares cholinergic systemic hazards.

Formulation and common adult teaching dose

• Oral capsules; a common adult regimen for Sjögren-related xerostomia is 30 mg three times daily.
• Use the product-specific hepatic/renal advice and do not treat dose as interchangeable with pilocarpine.

Indications

• Symptomatic dry mouth in Sjögren syndrome where licensed/available.

Common side effects

• Excess sweating: often the most limiting adverse effect.
• Nausea, diarrhoea and abdominal discomfort: increased GI smooth-muscle activity/secretions.
• Rhinitis and urinary frequency: mucosal/bladder muscarinic stimulation.

Serious adverse effects

• Bronchospasm: can worsen asthma or chronic obstructive disease.
• Cardiac rhythm/BP effects: susceptible patients may not tolerate autonomic haemodynamic changes.
• Visual disturbance: may impair driving or machine operation.

Contraindications and cautions

• Contraindicated in uncontrolled asthma and where miosis is undesirable, including narrow-angle glaucoma without specialist direction.
• Use caution with cardiovascular disease, active peptic ulcer and airway disease.

Interactions

• Antimuscarinics oppose benefit; other cholinomimetics/AChE inhibitors add toxicity.
• Review medicines that lower pulse/BP before starting.

Monitoring and counselling

• Monitor symptom relief, fluid loss from sweating, GI effects, wheeze and visual symptoms.
• Maintain hydration and report breathlessness, severe diarrhoea or troublesome blurred vision.

Carbachol

Mechanism and receptor profile

• Direct cholinergic agonist with muscarinic and nicotinic activity; resistant to AChE.
• Its major clinical use is topical/intraocular, not routine systemic prescribing.

Formulation and common adult teaching dose

• Intraocular solution used by ophthalmology to produce miosis during eye surgery; concentration/volume are product- and procedure-specific.
• Do not prescribe systemic carbachol as a routine substitute for bethanechol or pilocarpine.

Indications

• Induction of miosis during ophthalmic surgery and selected ophthalmology protocols.

Common side effects

• Ocular irritation/blurred vision: local application and pupil constriction affect vision.
• Brow ache/headache: ciliary muscle contraction may cause discomfort.

Serious adverse effects

• Systemic cholinergic effects: bradycardia, bronchospasm or sweating are possible after absorption.
• Marked ocular inflammation/pressure changes: require ophthalmic assessment rather than repeated self-treatment.

Contraindications and cautions

• Use only under ophthalmology supervision, with special caution in asthma, cardiovascular disease and retinal disease.

Interactions

• Other cholinergic medicines add systemic effects; antimuscarinics oppose miosis.

Monitoring and counselling

• Monitor ocular response and systemic pulse/respiratory symptoms when clinically indicated.
• Advise prompt review for severe eye pain, vision loss, wheeze or fainting.

Nicotine replacement therapy (NRT)

Mechanism and receptor profile

• Agonist at neuronal nicotinic receptors, including reward-pathway receptors; reduces withdrawal by replacing nicotine without tobacco combustion products.
• It is not harmless nicotine, but it is much safer than continued smoking for most people when used correctly.

Formulations and common adult teaching doses

• Patch, gum, lozenge, inhalator/spray vary by market. A common patch course begins at 21 mg/24 h daily for heavier smokers, then tapers.
• Gum/lozenges are commonly 2 mg or 4 mg, used to a maximum specified by the product. Combine long-acting patch with short-acting rescue NRT when appropriate.

Indications

• Tobacco-dependence treatment alongside behavioural support and a quit plan.

Common side effects

• Local irritation: skin reactions with patches; mouth/throat irritation or hiccups with oral products.
• Nausea and dyspepsia: often indicate swallowing nicotine-containing saliva or excessive dose.
• Insomnia/vivid dreams: may occur with 24-hour patches.
• Palpitations: nicotine stimulates autonomic ganglia/adrenal pathways.

Serious adverse effects

• Nicotine toxicity: vomiting, sweating, tremor, tachycardia, confusion or seizures after excessive exposure/accidental ingestion.
• Acute cardiovascular symptoms: chest pain or significant arrhythmia needs assessment, not simply a dose increase.

Contraindications and cautions

• Use a tailored plan in pregnancy, adolescents and recent acute cardiovascular illness; stopping smoking remains a priority and needs clinician support.
• Keep all nicotine products away from children and pets.

Interactions

• Smoking cessation—not nicotine itself—changes CYP1A2 induction and may increase concentrations of drugs such as clozapine, olanzapine and theophylline; arrange medication review.
• Avoid continued smoking while using high-dose NRT because nicotine excess can occur.

Monitoring and counselling

• Set a quit date, document cigarettes/day and review cravings, withdrawal and adverse effects.
• Use gum with a “chew and park” technique; rotate patch sites and remove patches before MRI if the product contains metal.

Varenicline

Mechanism and receptor profile

• Partial agonist at α4β2 neuronal nicotinic receptors: reduces withdrawal/craving while blocking much of the reward from smoked nicotine.
• It is not a full muscarinic agonist and does not treat cholinergic bladder/eye disease.

Formulation and common adult teaching dose

• Oral treatment commonly starts 0.5 mg once daily, then 0.5 mg twice daily, then 1 mg twice daily from day 8; renal adjustment is essential.
• Start before the planned quit date according to the programme/product schedule.

Indications

• Smoking cessation with behavioural support.

Common side effects

• Nausea: common and often improved by taking after food with water.
• Insomnia/vivid dreams: sleep effects are frequently reported.
• Headache and constipation: may affect adherence.

Serious adverse effects

• Neuropsychiatric change: new agitation, depressed mood, suicidal thoughts or unusual behaviour requires urgent assessment; distinguish drug effects from nicotine withdrawal but act on safety.
• Seizures or severe skin reaction: rare but require immediate discontinuation and urgent care.

Contraindications and cautions

• Check renal function and adjust dose; review seizure history and significant mental-health history with a support plan.
• Use pregnancy-specific cessation guidance rather than routine adult assumptions.

Interactions

• Few major pharmacokinetic interactions, but alcohol tolerance/behaviour can change in some patients.
• As with NRT, stopping smoking may change levels of CYP1A2-substrate medicines.

Monitoring and counselling

• Review quit progress, mood, sleep, nausea and adherence within the first weeks.
• Take after food with water; seek urgent help for severe mood change, self-harm thoughts, seizure or serious rash.

Neostigmine

Mechanism and receptor profile

• Reversible carbamate AChE inhibitor that increases ACh at muscarinic sites and the neuromuscular junction; it does not cross the blood–brain barrier.
• Also has some direct nicotinic action at the NMJ, improving transmission in myasthenia gravis.

Formulations and common adult teaching doses

• Oral and injectable forms exist. For reversal of non-depolarising neuromuscular block, a common monitored range is 0.03–0.07 mg/kg IV with an antimuscarinic such as glycopyrrolate/atropine, guided by train-of-four monitoring.
• MG and ileus/urinary-retention regimens are product- and specialist-specific; do not use an anaesthetic reversal dose as a ward prescription.

Indications

• Reversal of residual non-depolarising neuromuscular blockade by trained anaesthesia providers.
• Myasthenia gravis and selected postoperative ileus/urinary retention protocols.

Common side effects

• Abdominal cramps, diarrhoea and salivation: excess muscarinic ACh.
• Bradycardia: M₂ stimulation is why antimuscarinic co-administration is needed for reversal.
• Muscle twitching: increased ACh at NMJ can cause fasciculation.

Serious adverse effects

• Bronchospasm/bronchorrhoea: secretions and smooth-muscle contraction may impair ventilation.
• Cholinergic crisis: weakness can worsen from depolarisation block, with severe muscarinic toxicity.
• Severe bradyarrhythmia: may cause hypotension or arrest in susceptible patients.

Contraindications and cautions

• Avoid in mechanical GI/urinary obstruction and peritonitis; use caution in asthma, bradyarrhythmia, peptic ulcer and coronary disease.
• Never give reversal blindly: assess residual block with a peripheral nerve stimulator and recovery of ventilation.

Interactions

• Other cholinergic drugs add toxicity; antimuscarinics oppose muscarinic effects.
• Aminoglycosides, magnesium and some antiarrhythmics can worsen neuromuscular weakness; check anaesthetic/neuromuscular interactions.

Monitoring and counselling

• For reversal: continuous ECG, pulse oximetry, ventilation and train-of-four monitoring.
• For longer treatment: monitor muscle strength, secretions, bowel function, pulse and signs of under- versus over-treatment.

Pyridostigmine

Mechanism and receptor profile

• Reversible quaternary AChE inhibitor that predominantly acts peripherally and does not meaningfully enter the CNS.
• Raises ACh at the NMJ, improving fatigable weakness in myasthenia gravis; it also produces muscarinic GI/bladder effects.

Formulations and common adult teaching doses

• Immediate-release 60-mg tablets, liquid and extended-release products exist.
• A common starting clinical range is 30–60 mg orally every 4–6 hours while awake, timed to functional need; total daily dose is individualised. Do not crush extended-release tablets.

Indications

• Symptomatic treatment of myasthenia gravis.
• Specialist-selected uses such as reversal or autonomic disorders vary by protocol.

Common side effects

• Diarrhoea, cramps and nausea: muscarinic gut stimulation.
• Salivation/sweating: increased exocrine secretion.
• Fasciculation: excess ACh at the NMJ.

Serious adverse effects

• Cholinergic crisis: excess dose causes worsening weakness plus muscarinic signs; can mimic myasthenic crisis.
• Bronchial secretions/bronchospasm: may precipitate respiratory failure.
• Bradycardia/heart block: clinically important, especially with cardiac disease or nodal blockers.

Contraindications and cautions

• Avoid in mechanical urinary/GI obstruction; use caution in asthma, bradycardia, peptic ulcer and renal impairment.
• Worsening weakness requires assessment for infection, missed treatment, myasthenic crisis and cholinergic crisis—not unsupervised dose escalation.

Interactions

• Aminoglycosides, magnesium, quinine/quinidine and some anaesthetics can worsen NMJ transmission.
• Antimuscarinics may reduce secretions/GI effects but can mask toxicity; coordinate with the prescriber.

Monitoring and counselling

• Time doses around meals/activities when instructed, and monitor speech, swallowing, respiratory effort and objective strength.
• Urgent review is needed for increasing weakness, dyspnoea, inability to clear secretions or severe diarrhoea/sweating.

Physostigmine

Mechanism and receptor profile

• Reversible tertiary AChE inhibitor that crosses the blood–brain barrier, increasing ACh centrally and peripherally.
• This CNS penetration explains its selective role in severe antimuscarinic delirium and its greater seizure/bradyarrhythmia risk.

Formulation and common adult teaching dose

• IV injection administered slowly by clinicians experienced in toxicology; a common adult teaching dose is 0.5–2 mg IV slowly, with cautious repeat dosing only under protocol and monitoring.
• It is not a routine “antidote for any confusion”; toxicology advice and ECG assessment are essential.

Indications

• Selected severe, confirmed antimuscarinic toxicity with delirium when supportive care and contraindication review permit.

Common side effects

• Nausea, salivation and sweating: peripheral muscarinic excess.
• Bradycardia: vagal muscarinic stimulation.
• Abdominal cramping: increased GI smooth-muscle activity.

Serious adverse effects

• Seizures: CNS ACh excess or inappropriate use can provoke convulsions.
• Asystole/severe bradyarrhythmia: potentially fatal without monitoring/resuscitation readiness.
• Bronchospasm: especially hazardous in underlying airway disease.

Contraindications and cautions

• Avoid in suspected tricyclic-antidepressant poisoning with conduction delay/QRS widening, significant bradycardia or mechanical obstruction unless a toxicologist directs otherwise.
• Obtain ECG and assess for seizures, asthma and mixed overdose before considering it.

Interactions

• Other cholinergic agents add toxicity; drugs that slow conduction increase risk.
• Antimuscarinics such as atropine are rescue treatment for troublesome muscarinic toxicity, guided by the team.

Monitoring and counselling

• Continuous ECG, pulse oximetry, BP and seizure readiness are mandatory.
• Use only in a monitored resuscitation-capable setting with toxicology/critical-care support.