What are the side effects of Prazosin?

Prazosin Side Effects

Prazosin's most critical side effect is first-dose syncope with sudden loss of consciousness occurring in approximately 1% of patients, caused by excessive orthostatic hypotension from acute alpha-1 receptor blockade, which mandates starting at 1 mg and gradual titration. 1

Cardiovascular Side Effects (Most Serious)

First-Dose Phenomenon

• Syncope occurs in approximately 1% of patients given initial doses of 2 mg or greater, typically within 30-90 minutes of the first dose, and can be preceded by severe tachycardia with heart rates of 120-160 bpm 1
• The mechanism involves acute vasodilation from alpha-1 blockade causing excessive postural hypotension 2
• This risk is minimized by starting with 1 mg at bedtime, slow dose titration, and cautious introduction of additional antihypertensives 1, 3

Other Cardiovascular Effects

• Orthostatic hypotension occurs in 1-4% of patients and may be exacerbated when combined with beta-blockers like propranolol 1
• Palpitations affect 5.3% of patients 1
• Edema, dyspnea, tachycardia, angina pectoris, and hypotension occur less frequently 1

Common Side Effects (>5% Incidence)

• Dizziness (10.3%) - the most frequent adverse effect 1
• Headache (7.8%) 1
• Drowsiness (7.6%) 1
• Lack of energy (6.9%) 1
• Weakness (6.5%) 1
• Nausea (4.9%) 1

Genitourinary Side Effects

Priapism (Serious but Rare)

• Prolonged erections and priapism have been reported in post-marketing surveillance; any erection lasting longer than 4 hours requires immediate medical attention to prevent permanent penile tissue damage and loss of potency 1
• Urinary frequency, incontinence, and impotence have also been reported 1

Central Nervous System Effects

• Vertigo, depression, and nervousness occur in 1-4% of patients 1
• Rare effects include paresthesia and hallucinations (<1%) 1
• Post-marketing reports suggest potential worsening of pre-existing narcolepsy, though causality is uncertain 1

Gastrointestinal Effects

• Vomiting, diarrhea, and constipation occur in 1-4% of patients 1
• Rare cases of abdominal discomfort, liver function abnormalities, and pancreatitis have been reported 1

Other Notable Side Effects

• Intraoperative Floppy Iris Syndrome (IFIS) during cataract surgery has been associated with alpha-1 blocker therapy 1
• Dermatologic reactions including rash (1-4%), pruritus, alopecia, and lichen planus 1
• EENT effects: blurred vision, nasal congestion, dry mouth, epistaxis, tinnitus 1
• Rare reports of positive ANA titer, gynecomastia, and allergic reactions 1

Clinical Management Strategies

Minimizing First-Dose Effects

• Always initiate therapy with 1 mg capsules at bedtime - the 2 mg and 5 mg capsules are not indicated for initial therapy 1
• Consider withholding diuretics for 1 day before starting prazosin 3
• Warn patients to avoid situations where injury could occur if syncope develops during initiation 1

Monitoring Requirements

• Blood pressure monitoring is essential, particularly after initial dosing and with each significant dose increase 4, 5
• If syncope occurs, place patient in recumbent position and provide supportive treatment 1

Long-Term Considerations

• Most side effects disappear with continued therapy or are tolerated without dose reduction 1
• Fluid retention may rarely develop during long-term therapy, potentially requiring diuretic addition 3
• Sexual dysfunction is uncommon compared to other antihypertensive agents 3