Makindo Medical Notes

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.

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