Prazosin 💊
Related Subjects:
| Hypertension
| Chronic Heart Failure
| Heart Failure and Pulmonary Oedema
| Loop Diuretics
| Entresto Sacubitril with Valsartan
| Ivabradine
| Furosemide
| ACE Inhibitors
| CRT Pacemaker
⚠️ First-dose hypotension is common. Always give the first dose of prazosin at bedtime and start with the lowest possible dose.
ℹ️ About
- Prazosin is a selective alpha-1 adrenergic receptor antagonist.
- Causes vasodilation of arterioles and venules → reduced systemic vascular resistance and blood pressure.
- Also relaxes smooth muscle in the bladder neck and prostate, improving urinary outflow in BPH.
- BNF link here
Mode of Action 🧬
- Blocks post-synaptic alpha-1 adrenergic receptors in vascular smooth muscle → vasodilation → ↓ preload & afterload.
- In bladder/prostate → ↓ resistance at bladder neck and urethra → improved flow in BPH.
- Unlike non-selective alpha-blockers, prazosin does not significantly affect presynaptic alpha-2 receptors → less reflex tachycardia.
Indications 📋
- Hypertension: Second-line/adjunct therapy, especially in resistant cases.
- Benign prostatic hyperplasia (BPH): Relieves lower urinary tract symptoms.
- Raynaud’s phenomenon: Improves digital blood flow.
- Heart failure: Historically used to reduce preload/afterload, but now rarely employed given superior modern therapies (ACEi/ARB/ARNI, beta-blockers, MRA, SGLT2i).
Dose 💊
- Hypertension: Start 500 micrograms bd–tds for 3–7 days → increase to 1 mg bd–tds. Max 20 mg/day.
- Heart failure: Start 500 micrograms bd–tds; caution if >4 mg/day.
- Short half-life → requires multiple daily dosing (contrast with doxazosin once daily).
Pharmacokinetics
- Half-life: 2–3 hours (shorter than doxazosin).
- Extensive hepatic metabolism.
- Requires bd/tds dosing for BP control.
Interactions ⚠️
- Other antihypertensives, nitrates, or PDE5 inhibitors → additive hypotension.
- NSAIDs may blunt antihypertensive effect.
Cautions
- First-dose phenomenon: Profound postural hypotension/syncope → start at night.
- Cataract surgery: Risk of intraoperative floppy iris syndrome (also with tamsulosin). Inform ophthalmic surgeon if patient on alpha-blockers.
Contraindications 🚫
- Symptomatic hypotension.
- Relative caution in pregnancy and lactation (limited data).
Side Effects 🩺
- Dizziness, headache, postural hypotension, syncope.
- Fatigue, weakness, palpitations, oedema.
- Less common: anxiety, sweating, arthralgia.
- Rare: hepatic dysfunction.
Clinical Pearls ✨
- 💡 Prazosin vs doxazosin: Prazosin shorter half-life (requires multiple daily doses); doxazosin longer half-life (once daily).
- ⚡ First-dose hypotension: Common exam favourite - counsel patients to take first dose at bedtime.
- 👁️ Ophthalmology pearl: Floppy iris syndrome in cat
