The most common reactions associated with prazosin therapy are postural dizziness (11%), nausea (9.5%), drowsiness (8.7%), headache (8.4%), palpitations (6.6%), dry mouth (5.6%), weakness (4.6%) and fatigue/malaise (4.5%). In most instances side effects have disappeared with continued therapy or have been tolerated with no decrease in dose of drug. The following reactions have also been observed during prazosin administration, some of them rarely (1 to 4%).
vomiting, diarrhea, constipation, abdominal discomfort and/or pain, liver function abnormalities, pancreatitis.
syncope (see Warnings), edema, dyspnea, orthostatic hypotension, tachycardia.
nervousness, vertigo, depression, paresthesia, hallucinations.
rash, pruritus, alopecia, lichen planus.
urinary frequency, incontinence, impotence, priapism.
blurred vision, reddened sclera, epistaxis, tinnitus, nasal congestion, dry mouth.
diaphoresis, fever, positive ANA titer, arthralgia.
Single reports of pigmentary mottling and serious retinopathy, and a few reports of cataract development have been reported. In these instances, the exact causal relationship has not been established because the baseline observations were frequently inadequate.
In more specific slit lamp and funduscopic studies which included adequate baseline examinations, no drug-related abnormal ophthalmological findings have been reported.
Published reports indicate that the administration of prazosin in cases of pre-existing narcolepsy increases the frequency of cataplectic attacks; a causal relationship has not been established in all cases.