Doxazosin (Cardura)
Alpha-1 Blocker · Treats enlarged prostate (BPH) and high blood pressure
What is Doxazosin?
Doxazosin is a selective alpha-1 adrenergic receptor antagonist (alpha-blocker) that was first approved in the United States in 1990. It belongs to the same drug class as tamsulosin, though doxazosin is less selective for the prostate and more commonly used when both BPH and hypertension need to be treated simultaneously.
An extended-release formulation (Cardura XL) is available and delivers the drug gradually over 24 hours, which reduces the first-dose orthostatic hypotension risk compared to the immediate-release tablet.
What is Doxazosin Used For?
Doxazosin has two primary FDA-approved indications:
- Benign prostatic hyperplasia (BPH): Improves urine flow and reduces symptoms like weak stream, urgency, and nighttime urination by relaxing smooth muscle in the prostate and bladder neck
- Hypertension: Lowers blood pressure by dilating blood vessels through alpha-1 receptor blockade
Because it addresses both conditions simultaneously, doxazosin is a logical choice for older men who have both an enlarged prostate and high blood pressure.
How Does Doxazosin Work?
Alpha-1 adrenergic receptors are found in the smooth muscle of blood vessel walls, the prostate gland, and the bladder neck. When the neurotransmitter norepinephrine binds to these receptors, it causes the muscle to contract.
Doxazosin blocks these receptors, preventing contraction. In blood vessels, this causes vasodilation and reduced blood pressure. In the prostate and bladder neck, it relaxes the muscular component of BPH, reducing the obstruction to urine flow and improving urinary symptoms.
Side Effects
Common Side Effects
- Dizziness and lightheadedness (especially when standing up)
- Fatigue and drowsiness
- Headache
- Nasal congestion
- Swelling in ankles or feet (edema)
Serious Side Effects
- Orthostatic hypotension (severe drop in blood pressure on standing)
- Syncope (fainting) — most common after first dose
- Intraoperative floppy iris syndrome (IFIS) — important for cataract surgery; inform your ophthalmologist
- Priapism (prolonged erection) — rare
Drug Interactions
Doxazosin's blood-pressure-lowering effects can be amplified by other medications, leading to excessive hypotension.
| Drug/Substance | Interaction Type | Effect |
|---|---|---|
| PDE5 inhibitors (sildenafil, tadalafil) | Additive hypotension | Severe blood pressure drop — concurrent use is generally contraindicated |
| Other antihypertensives | Additive hypotension | Enhanced blood pressure lowering |
| Beta-blockers | Additive hypotension | Increased risk of orthostatic hypotension |
| NSAIDs (ibuprofen, naproxen) | Reduced efficacy | NSAIDs can blunt antihypertensive effects |
| CYP3A4 inhibitors (ketoconazole) | Increased drug levels | Higher doxazosin exposure and hypotension risk |
Who Should Not Take Doxazosin?
- Patients with a history of orthostatic hypotension or syncope
- Men planning cataract surgery (IFIS risk — inform surgeon)
- Patients taking PDE5 inhibitors (sildenafil, tadalafil) for erectile dysfunction — risk of severe hypotension
- Women with stress urinary incontinence (alpha-blockers can worsen it)
Frequently Asked Questions
What is doxazosin used for?
Doxazosin is used to treat benign prostatic hyperplasia (BPH, enlarged prostate) to improve urine flow and urinary symptoms, and to lower blood pressure in hypertension. It is useful when both conditions are present in the same patient.
How is doxazosin different from tamsulosin?
Both are alpha-1 blockers for BPH, but tamsulosin is more selective for the prostate (alpha-1A receptors) and has less blood-pressure-lowering effect. Doxazosin is less selective and therefore more useful when hypertension also needs to be treated.
What is the first-dose effect?
The first-dose effect refers to a significant drop in blood pressure that can occur after the first dose of doxazosin, particularly when standing up. This can cause dizziness or fainting. Taking the first dose at bedtime minimizes the risk of a dangerous fall.
Can I take doxazosin with Viagra (sildenafil)?
This combination should generally be avoided. Both doxazosin and sildenafil lower blood pressure, and combining them can cause a severe, potentially dangerous drop in blood pressure. Always inform your doctor about all medications you are taking.