Antiplatelet drugs: prevent platelet-rich arterial thrombosis
Antiplatelet drugs reduce platelet activation, adhesion and aggregation; they do not dissolve an established clot. They are central to secondary prevention after arterial events (myocardial infarction, ischaemic stroke/TIA and peripheral arterial disease) and to acute coronary syndrome (ACS) and percutaneous coronary intervention (PCI). Prescribing always balances thrombosis prevention against bleeding risk, diagnosis, renal/hepatic function, interacting medicines and the planned duration.
Safety first
Do not stop aspirin, clopidogrel, prasugrel or ticagrelor after an ACS or coronary stent without the treating cardiology/medical team. Premature interruption can cause stent thrombosis, myocardial infarction or death. Active major bleeding, suspected intracranial bleeding, haemodynamic instability, or urgent surgery requires immediate senior assessment and protocol-led management.
Lesson map
Primary haemostasis and platelet plug formation.
COX-1, P2Y12, PDE and GP IIb/IIIa inhibitors.
ACS/PCI, secondary prevention, stroke and PAD.
Bleeding, interactions, procedures and counselling.
1. Why antiplatelet drugs work: primary haemostasis
| Stage | What happens | Drug relevance |
|---|---|---|
| Vessel injury | Collagen and von Willebrand factor (vWF) become exposed; platelets adhere through surface glycoproteins. | Antiplatelets chiefly target the platelet response, not fibrin production. |
| Activation | Platelets change shape and release ADP, thromboxane A2 (TXA2), serotonin and other mediators. | Aspirin reduces TXA2; P2Y12 inhibitors block ADP signalling. |
| Aggregation | Activated GP IIb/IIIa binds fibrinogen, cross-linking platelets into a platelet plug. | GP IIb/IIIa inhibitors block this final common pathway. |
| Secondary haemostasis | Thrombin generates fibrin to stabilise the clot. | Anticoagulants mainly act here; combination therapy substantially increases bleeding risk. |
Clinical distinction: platelet-rich “white” thrombi are typical of high-shear arterial disease (coronary, cerebral and peripheral arteries); fibrin-rich venous thrombi are principally managed with anticoagulants. The distinction is useful, but real patients may need both drug groups for separate indications.
2. Classification and key mechanisms
| Class | Examples | Mechanism and practical point |
|---|---|---|
| COX-1 inhibitor | Aspirin | Irreversibly acetylates platelet COX-1, reducing TXA2. Effect lasts for the affected platelet’s lifespan. |
| P2Y12 receptor inhibitors | Clopidogrel, prasugrel, ticagrelor; IV cangrelor | Block ADP-dependent platelet activation and GP IIb/IIIa activation. Clopidogrel/prasugrel are prodrugs; ticagrelor is direct and reversible. |
| Phosphodiesterase/adenosine pathway agents | Dipyridamole; cilostazol | Increase platelet cAMP and reduce aggregation. Indication varies; cilostazol is avoided in heart failure. |
| GP IIb/IIIa inhibitors | Abciximab, eptifibatide, tirofiban | IV specialist drugs blocking fibrinogen-mediated platelet cross-linking; used selectively around high-risk PCI/ACS. |
| PAR-1 antagonist | Vorapaxar | Blocks thrombin-mediated platelet activation; specialist use only because of serious bleeding risk. |
3. Aspirin
Mechanism and uses
At antiplatelet doses, aspirin irreversibly suppresses platelet TXA2 production. Platelets cannot replace COX-1, whereas endothelial cells can resynthesise it. Aspirin is commonly used for secondary prevention of myocardial infarction, ischaemic stroke/TIA and peripheral arterial disease, and as one component of dual antiplatelet therapy (DAPT) for appropriate ACS/PCI patients. It is not a routine self-started medicine for everyone: primary-prevention benefit must be weighed against bleeding.
- Important adverse effects: dyspepsia, gastritis/ulceration, gastrointestinal bleeding, bruising, hypersensitivity/bronchospasm and rarely intracranial bleeding.
- Major cautions: active bleeding, previous serious aspirin hypersensitivity, peptic ulcer disease, asthma with NSAID sensitivity, severe liver disease and concurrent medicines that increase bleeding.
- Children and adolescents: avoid aspirin in viral illness unless a specialist indication exists because of Reye syndrome risk.
- Interaction pearl: ibuprofen and other NSAIDs can increase gastrointestinal bleeding; avoid self-medication and follow local advice on timing/alternatives.
4. P2Y12 inhibitors
| Medicine | High-yield pharmacology | Safety detail |
|---|---|---|
| Clopidogrel | Irreversible thienopyridine prodrug activated mainly through CYP2C19. Used alone when aspirin is unsuitable and with aspirin when DAPT is indicated. | Bleeding, GI upset/rash; rare thrombotic thrombocytopenic purpura/neutropenia. Omeprazole and esomeprazole may reduce activation—use a prescriber/pharmacist-approved gastroprotection plan. |
| Prasugrel | Irreversible thienopyridine with rapid, potent platelet inhibition; specialist ACS/PCI use. | Higher bleeding risk; generally contraindicated with prior stroke/TIA and used cautiously in older or low-weight patients under cardiology guidance. |
| Ticagrelor | Direct, reversible oral P2Y12 inhibitor, usually taken more than once daily according to the prescribed regimen. | Bleeding, dyspnoea and bradyarrhythmia can occur; review CYP3A interactions and adherence carefully. |
| Cangrelor | Rapid-onset, reversible IV P2Y12 inhibition. | Used in monitored specialist/PCI settings; bleeding is the key toxicity. |
Do not interchange P2Y12 agents or change their dose/duration casually. The indication, timing of ACS/PCI, bleeding risk, previous stroke/TIA, drug interactions and local protocol determine the choice.
5. Other antiplatelet agents
- Dipyridamole: increases extracellular adenosine and platelet cAMP. In some settings it is combined with aspirin for selected secondary stroke-prevention regimens. Headache, flushing and hypotension limit use; confirm the indication locally.
- Cilostazol: PDE-3 inhibitor with antiplatelet and vasodilator actions, used in selected intermittent claudication pathways. Avoid in heart failure.
- GP IIb/IIIa inhibitors: powerful IV agents used by specialist teams in selected high-risk PCI/ACS cases. Monitor for bleeding and acute thrombocytopenia; renal adjustment applies to some agents.
6. Core clinical indications
| Situation | Usual principle | Critical caution |
|---|---|---|
| Acute coronary syndrome | Aspirin plus a P2Y12 inhibitor is a major component of care alongside reperfusion/anticoagulation decisions. | Agent and duration are guideline-, procedure- and bleeding-risk-specific. |
| PCI/coronary stent | DAPT reduces stent thrombosis and recurrent MI. | Never stop either component early without the treating cardiology team. |
| Prior MI/PAD | Long-term single antiplatelet therapy is often used for secondary prevention. | Confirm diagnosis, bleeding history and concurrent anticoagulation. |
| Ischaemic stroke/TIA | Antiplatelet treatment is used for non-cardioembolic disease; short-course DAPT may be used in selected acute pathways. | Atrial fibrillation-related stroke prevention usually requires anticoagulation, not antiplatelet substitution. |
| Mechanical/prosthetic valve or VTE | Antiplatelets may occasionally have a specific additional role. | They do not replace required anticoagulation; specialist direction is essential. |
DAPT rule
DAPT means aspirin + a P2Y12 inhibitor. It is not automatically lifelong and it is not interchangeable with anticoagulation. Duration must be documented, reviewed and individualised by the relevant guideline/team. Adding an anticoagulant (triple therapy) markedly increases bleeding risk and should follow a defined specialist plan.
7. Pre-treatment and follow-up checklist
- Confirm the indication: ACS, PCI/stent, secondary prevention, stroke/TIA or PAD.
- Record bleeding history: previous GI or intracranial bleed, ulcer disease, anaemia, thrombocytopenia, liver disease, alcohol excess and recent surgery.
- Review all medicines: anticoagulants, NSAIDs, corticosteroids, SSRIs/SNRIs, other antiplatelets, herbal products and PPI selection with clopidogrel.
- Check baseline FBC/platelets; consider renal/liver function and pregnancy status where clinically relevant.
- Document the drug, reason, intended duration, review date and stop plan. For stents, record the PCI/cardiology contact pathway.
- At review, ask about adherence, bruising, black stool, haematuria, epistaxis, breathlessness (ticagrelor), dyspepsia and planned procedures.
8. Adverse effects, interactions and procedures
| Problem | What to recognise | Action principle |
|---|---|---|
| Minor bleeding | Easy bruising, gum bleeding, epistaxis, prolonged bleeding from cuts or heavier menses. | Assess severity, adherence and interacting medicines; do not stop a recent-stent regimen independently. |
| Possible major bleed | Haematemesis, melaena, fresh rectal bleeding, haematuria, haemoptysis, collapse, severe headache/neurological deficit or persistent heavy bleeding. | Urgent emergency assessment; withhold/reversal decisions are senior, indication-specific and protocol-led. |
| GI protection | Older age, prior ulcer/bleed or combined antithrombotic therapy increase risk. | Consider prescribed gastroprotection. For clopidogrel, avoid omeprazole/esomeprazole unless the prescriber specifically directs otherwise. |
| Dental/surgical/neuraxial procedure | Bleeding risk must be balanced against thrombosis risk. | Tell the proceduralist and prescribing team early; never give generic stop dates from a summary page. |
| Thrombocytopenia | New petechiae, bleeding or unexpectedly low platelets, especially after IV GP IIb/IIIa treatment. | Stop-and-investigate decisions require urgent clinical review. |
9. Patient counselling and red flags
- Take exactly as prescribed; use a current medicines list or alert card where available.
- Tell every clinician, dentist and pharmacist about the antiplatelet medicine before a procedure or new drug.
- Do not add over-the-counter aspirin, ibuprofen/naproxen, traditional/herbal remedies or alcohol excess without advice.
- Seek urgent care for black stools, vomiting blood, severe/persistent headache, new weakness/speech difficulty, unexplained fainting, blood in urine, major trauma or bleeding that will not stop.
- For a missed dose, follow the medicine-specific leaflet/prescriber advice; do not double doses.
10. High-yield clinical cases
A patient wants to stop aspirin and clopidogrel because of bruising. Explain that bruising needs assessment, but stopping DAPT without cardiology advice can precipitate stent thrombosis. Escalate promptly.
Antiplatelets do not automatically substitute for anticoagulation in atrial fibrillation. Identify the stroke mechanism and involve the appropriate team.
Check for ibuprofen/aspirin analgesia and omeprazole/esomeprazole. Explain bleeding/interaction risk and obtain pharmacist-prescriber advice.
Treat as potential significant GI bleeding: assess ABCs, FBC/group-and-save and urgent senior/GI pathway; decisions about antiplatelets depend on the indication and haemostasis plan.
Exam and OSCE essentials
- Define antiplatelets and distinguish primary from secondary haemostasis.
- Name the four common classes and one mechanism/example for each.
- Explain why aspirin’s effect persists for the platelet lifespan.
- State that DAPT = aspirin + P2Y12 inhibitor, commonly used after ACS/PCI, with duration set by protocol/specialist assessment.
- Identify bleeding, active bleeding, drug interactions and planned procedures as safety priorities.
- Ask specifically about recent stent placement before advising treatment interruption.
Further study
Course reference: Antiplatelet Drugs—SlideShare. Expand this lesson with current local/national guidance, the NICE acute coronary syndromes recommendations, MedlinePlus clopidogrel information, and the ACC ACS antiplatelet overview. Drug selection and treatment duration must follow the current local protocol and responsible specialist.
