Prazosin Side Effects

Medically reviewed by Drugs.com. Last updated on Mar 31, 2025.

Applies to prazosin: oral capsule.

Precautions

It is very important that your doctor check your progress at regular visits to make sure that this medicine is working properly. Blood and urine tests may be needed to check for unwanted effects.

Dizziness, lightheadedness, or fainting may occur after you use this medicine, especially when you get up from a lying or sitting position. Getting up slowly may help lessen this problem. If you feel dizzy, lie down so you do not faint. Then sit for a few moments before standing to prevent the dizziness from returning.

You may also have dizziness, lightheadedness, or fainting if you drink alcohol, stand for a long time, exercise or if the weather is hot. While you are taking this medicine, be careful to limit the amount of alcohol you drink. Drink plenty of fluids if you exercise or sweat more than usual while you are using this medicine.

Prazosin may cause some people to become dizzy, drowsy, or less alert than they are normally. Make sure you know how you react to this medicine before you drive, use machines, or do anything else that could be dangerous if you are dizzy, drowsy, or are not alert.

Make sure any doctor or dentist who treats you knows that you are using this medicine. This medicine may affect the results of certain medical tests.

Call your doctor right away if you have a prolonged erection of the penis while you are using this medicine. It must be treated right away to prevent permanent impotence.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

Serious side effects of prazosin

Along with its needed effects, prazosin may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking prazosin:

Other side effects of prazosin

Some side effects of prazosin may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

For healthcare professionals

Applies to prazosin: compounding powder, oral capsule.

General adverse events

The most common adverse reactions were postural hypotension, dizziness, headache, drowsiness, lack of energy, weakness, palpitations, and nausea.^[Ref]

Cardiovascular

• Very common (10% or more): Postural hypotension (up to 14%)
• Common (1% to 10%): Palpitations, tachycardia, orthostatic hypotension
• Uncommon (0.1% to 1%): Angina pectoris
• Rare (less than 0.1%): Bradycardia, flushing, hypotension, vasculitis
• Frequency not reported: Angina exacerbation, myocardial infarction^[Ref]

Nervous system

• Very common (10% or more): Dizziness (up to 10.3%)
• Common (1% to 10%): Drowsiness, headache, faintness, syncope, vertigo
• Uncommon (0.1% to 1%): Paresthesia, tinnitus
• Rare (less than 0.1%): Narcolepsy worsened^[Ref]

Other

• Common (1% to 10%): Lack of energy, weakness, asthenia
• Rare (less than 0.1%): Fever, pain
• Frequency not reported: Malaise^[Ref]

Gastrointestinal

• Common (1% to 10%): Constipation, diarrhea, dry mouth, nausea, vomiting
• Uncommon (0.1% to 1%): Abdominal discomfort and/or pain
• Rare (less than 0.1%): Pancreatitis^[Ref]

Psychiatric

• Common (1% to 10%): Depression, nervousness
• Uncommon (0.1% to 1%): Insomnia
• Rare (less than 0.1%): Hallucinations^[Ref]

Respiratory

• Common (1% to 10%): Dyspnea, nasal congestion, epistaxis^[Ref]

Ocular

• Common (1% to 10%): Blurred vision, sclera reddened
• Uncommon (0.1% to 1%): Eye pain
• Frequency not reported: Pigmentary mottling, serious retinopathy, cataract development, cataract disappearance^[Ref]

Genitourinary

• Common (1% to 10%): Urinary frequency, impotence
• Rare (less than 0.1%): Incontinence, priapism^[Ref]

Dermatologic

• Common (1% to 10%): Rash
• Uncommon (0.1% to 1%): Diaphoresis, pruritus, urticaria
• Rare (less than 0.1%): Alopecia, lichen planus^[Ref]

Metabolic

• Common (1% to 10%): Edema^[Ref]

Musculoskeletal

• Uncommon (0.1% to 1%): Arthralgia^[Ref]

Immunologic

• Rare (less than 0.1%): Allergic reaction, positive antinuclear antibody titer^[Ref]

Endocrine

• Rare (less than 0.1%): Gynecomastia^[Ref]

Hepatic

• Rare (less than 0.1%): Liver function abnormalities^[Ref]

See also:

Further information

Prazosin side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Note: Medication side effects may be underreported. If you are experiencing side effects that are not listed, submit a report to the FDA by following this guide.

Medical Disclaimer