Prazosin
A short-acting alpha-1 blocker used for high blood pressure and, in the UK, for urinary symptoms of BPH and Raynaud's phenomenon.
Drug card
- Class
- Alpha-1 blocker
- How it works
- Blocking alpha-1 receptors, relaxing blood vessels and prostate muscle
- Used for
- High blood pressure
- Urinary symptoms of BPH, as an add-on
- Raynaud's phenomenon
- Known for
- Fainting within hours of the first dose
- Dizziness on standing (postural hypotension)
- Its name
- -azosin means an alpha-1 blocker (prazosin type)
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does prazosin work?
The first quinazoline alpha-1 blocker. Blocking postsynaptic alpha-1 receptors widens mainly the arterioles, lowering peripheral resistance, usually without a reflex fast heart rate or a rise in renin. Its half-life is only 2 to 3 hours, so it is taken two or three times a day.
Like other alpha-1 blockers: They block alpha-1 adrenergic receptors on smooth muscle, so norepinephrine can no longer contract it. In arterioles and veins this lowers peripheral resistance and blood pressure; in the prostate, its capsule and the bladder neck the muscle relaxes and urine flows more easily. Tamsulosin favors the alpha-1A subtype that dominates the prostate and is not used for blood pressure.
Uses
What is prazosin used for?
- High blood pressure
- Urinary symptoms of BPH, as an add-on
- Raynaud's phenomenon
Side effects
Side effects of prazosin
- Fainting within hours of the first doseKnown forSerious
- Dizziness on standing (postural hypotension)Known for
- Headache
- Drowsiness
- Lack of energy and weakness
- Palpitations
- Depression and nervousness
- Edema and fluid retention
- Passing urine often, or incontinence
- Blocked nose
- Floppy iris syndrome during cataract surgery
- PriapismSerious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- First-dose fainting is the classic risk: start low, give the first dose in the evening and raise it slowly.
- A beta blocker, a diuretic or a PDE5 inhibitor adds to the fall in blood pressure.
- Avoid in men with BPH who have fainted while passing urine (micturition syncope).
- It can give false-positive urine tests for pheochromocytoma (VMA and MHPG).
Good to know: The shortest-acting of the three alpha-1 blockers here. In the UK it is also licensed for heart failure that has not responded to diuretics or digoxin.
Remember it
A way to remember it
FAINT
- F
- First-dose fall in blood pressure, with fainting
- A
- Abnormal ejaculation, most with tamsulosin
- I
- Intraoperative floppy iris syndrome in cataract surgery
- N
- Nasal stuffiness
- T
- Tiredness and weakness
Written for biofideX
Its name
-azosin means alpha-1 blocker (prazosin type)
Doxazosin and prazosin share it. Tamsulosin, an alpha-1 blocker used for an enlarged prostate, ends only in -osin; WHO lists it, with alfuzosin, as related.
- Every drug name stem
Its class
Alpha-1 blockers
Other alpha-1 blockers with a card:
Easily mixed up with central alpha-2 agonists, 5-alpha reductase inhibitors and beta blockers.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Prazosin hydrochloride capsules: label (DailyMed)
- Prazosin 0.5 mg tablets: SmPC (emc)
- Prazosin (NCI Thesaurus)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Blood pressure
Prazosin is one of 27 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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