Prazosin, Oral Capsule

Medically reviewed by University of Illinois-Chicago, Drug Information Group on November 2, 2016Written by University of Illinois-Chicago, Drug Information Group on October 14, 2017

Highlights for prazosin

  1. Prazosin oral capsule is available as a generic drug and a brand-name drug. Brand name: Minipress.
  2. Prazosin comes only as a capsule you take by mouth.
  3. Prazosin is used to treat high blood pressure (hypertension). It can be used alone or in combination with other medications to help lower your blood pressure.

Important warnings

  • Loss of consciousness: Because this medication lowers your blood pressure, prazosin may cause you to pass out, faint, or suddenly lose consciousness. This can happen within 30 to 90 minutes of your first dose of prazosin. It can also happen if your prazosin dose is increased too quickly, or if you’re already on a high dose of prazosin and you’re started on another blood pressure medication.
  • Prolonged erections: Prazosin may cause prolonged erections (priapism). If you have an erection that lasts more than 4 hours, call your doctor or seek emergency medical help right away. If this isn’t treated, it may result in erectile dysfunction or tissue scarring.
  • Intraoperative floppy iris syndrome: If you have cataracts and need surgery, be aware that prazosin may increase your risk for complications during surgery known as intraoperative floppy iris syndrome (IFIS). If you need eye surgery, let your eye doctor know that you’re taking prazosin. The doctor may give you medications before surgery or perform the surgery using different techniques to help decrease your risk for IFIS.

What is prazosin?

Prazosin is a prescription drug. It comes as an oral capsule.

Prazosin is available as the brand-name drug Minipress as well as in a generic version. Generic drugs usually cost less. In some cases, they may not be available in every strength or form as the brand-name version.

This drug may be used as part of a combination therapy. That means you need to take it with other drugs.

Why it's used

Prazosin is used to lower your blood pressure. If your blood pressure stays too high, it can put you at risk for serious health problems such as heart attack and stroke.

How it works

Your blood pressure can increase when your blood vessels constrict and get narrower. Prazosin works by relaxing your blood vessels so that blood will flow more easily through them. This will then make your blood pressure decrease.

Prazosin side effects

Prazosin oral capsule may cause dizziness and drowsiness after your first dose. Avoid driving or performing any hazardous tasks for the first 24 hours after taking this medication or when your dose is increased.

This drug can also cause other side effects.

More common side effects

The more common side effects of prazosin include:

  • dizziness
  • drowsiness
  • lightheadedness
  • headache
  • lack of energy
  • weakness
  • palpitations (feel like your heart is racing or fluttering)
  • nausea

If these effects are mild, they may disappear within a few days or a couple of weeks. If they’re more severe or don’t disappear, talk to your doctor or pharmacist.

Serious side effects

Call your doctor right away if you have serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency. Serious side effects and their symptoms can include the following:

  • loss of consciousness (this can occur if you stand up too quickly after sitting or lying down)
  • an erection lasting more than 4 hours
  • very fast heartbeat
  • swelling of your hands and feet
  • difficulty breathing
  • depression
  • allergic reactions, including:
    • skin rash
    • hives
    • swelling of the lips, face, or tongue
    • trouble breathing

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we can not guarantee that this information includes all possible side effects. This information is not a substitute for medical advice. Always discuss possible side effects with a healthcare provider who knows your medical history.

Prazosin may interact with other medications

Prazosin oral capsule can interact with other medications, vitamins, or herbs you may be taking. An interaction is when a substance changes the way a drug works. This can be harmful or prevent the drug from working well.

To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking. To find out how this drug might interact with something else you’re taking, talk to your doctor or pharmacist.

Examples of drugs that can cause interactions with prazosin are listed below.

Other blood pressure drugs

If you take prazosin together with other blood pressure drugs, they will decrease your blood pressure even more and may drop it to a dangerously low level. Your doctor can help you avoid this by reducing your prazosin dosage, adding any other blood pressure medications carefully, and then increasing your prazosin dosage slowly. Your doctor may also have you check your blood pressure more often.

Examples of other blood pressure drugs include:

  • metoprolol
  • atenolol
  • carvedilol
  • lisinopril
  • losartan
  • valsartan
  • hydrochlorothiazide
  • amlodipine
  • clonidine

Erectile dysfunction drugs

Erectile dysfunction drugs can lower your blood pressure. Taking any of these drugs with prazosin will lower your blood pressure even more and may drop it to a dangerously low level. Your doctor may choose to adjust your dosage or avoid this combination of drugs.

Examples of erectile dysfunction drugs include:

  • avanafil
  • sildenafil
  • tadalafil
  • vardenafil

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs interact differently in each person, we can not guarantee that this information includes all possible interactions. This information is not a substitute for medical advice. Always speak with your healthcare provider about possible interactions with all prescription drugs, vitamins, herbs and supplements, and over-the-counter drugs that you are taking.

Prazosin warnings

This drug comes with several warnings.

Allergy warning

Prazosin may cause a severe allergic reaction. Symptoms may include:

  • skin rash
  • hives
  • swelling of the lips, face, or tongue
  • trouble breathing

Call 911 or go to the nearest emergency room if you develop these symptoms.

Don’t take this drug again if you’ve ever had an allergic reaction to it. Taking it again could be fatal (cause death).

Alcohol interaction warning

If you drink alcohol while on prazosin, you may experience dizziness, lightheadedness, and fainting.

Warnings for people with certain health conditions

For people with heart problems: Use prazosin with caution if you have heart failure, swelling of your hands and feet (edema), tendency to get very dizzy or lightheaded when you stand from a lying or sitting position (orthostatic hypotension), or a type of chest pain called angina. Prazosin may worsen these conditions.

For people with eye problems: Prazosin can cause blurry vision and eye pain. If you already have eye problems such as cataracts and need surgery, prazosin may increase your risk for serious complications during eye surgery.

Warnings for other groups

For pregnant women: Prazosin is a category C pregnancy drug. That means two things:

  1. Research in animals has shown harmful effects to the fetus when the mother takes the drug.
  2. There haven’t been enough studies done in humans to be certain how the drug might affect the fetus.

Speak with your doctor if you’re pregnant or planning to become pregnant. Prazosin should be used during pregnancy only if the potential benefit justifies the potential risk to the pregnancy.

For women who are breastfeeding: Prazosin passes through breast milk. Talk to your doctor before breastfeeding while taking prazosin.

For seniors: If you’re 65 years or older and you take prazosin, you’re at an increased risk for a sudden drop in blood pressure when you stand up after sitting or lying down (orthostatic hypotension). This can cause dizziness and lightheadedness. Your doctor may start you on a lower dosage, monitor your blood pressure more closely, or choose a different blood pressure drug for you.

For children: Safety and effectiveness in children haven’t been established.

How to take prazosin

All possible dosages and forms may not be included here. Your dose, form, and how often you take it will depend on:

  • your age
  • the condition being treated
  • how severe your condition is
  • other medical conditions you have
  • how you react to the first dose

Drug forms and strengths

Generic: Prazosin

  • Form: oral capsule
  • Strengths: 1 mg, 2 mg, and 5 mg

Brand: Minipress

  • Form: oral capsule
  • Strengths: 1 mg, 2 mg, and 5 mg

Dosage for high blood pressure

Adult dosage (ages 18 years and older)

  • Typical dosage is 1 mg, taken either two or three times per day as instructed by your doctor.
  • Your doctor may slowly increase your dosage up to a maximum of 20 mg per day, taken in evenly divided doses.
  • Most people will need a dosage ranging from 6 mg to 15 mg daily, taken in evenly divided doses.

Child dosage (ages 0–17 years)

Safety and effectiveness for people younger than 18 years hasn’t been established.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we can not guarantee that this list includes all possible dosages. This information is not a substitute for medical advice. Always to speak with your doctor or pharmacist about dosages that are right for you.

Take as directed

Prazosin is used for long-term treatment. It comes with serious risks if you don’t take it as prescribed.

If you don’t take it at all: If you don’t take your prazosin and your blood pressure stays too high, it can put you at risk for serious health problems such as heart attack and stroke.

If you don’t take it on schedule: If you don’t take it on schedule, the drug will be less effective at maintaining a healthy blood pressure, which could put you at a higher risk for heart attack and stroke.

If you take too much: If you take too much prazosin, you may experience a large drop in blood pressure, leading to extreme dizziness, lightheadedness, fainting, or even loss of consciousness and shock. If you think you may have taken too much prazosin, call your doctor or get emergency medical help right away.

What to do if you miss a dose:If you forget to take your dose, take it as soon as you remember, unless you remember just a few hours before the time for your next dose. Then only take one dose.

Never try to catch up by taking two tablets at once. This could result in dangerous side effects, such as a large drop in blood pressure (which can lead to dizziness, lightheadedness, fainting, or even loss of consciousness).

How to tell if the drug is working: Your blood pressure should decrease.

Important considerations for taking prazosin

these considerations in mind if your doctor prescribes prazosin for you.

General

  • Prazosin comes as capsules that may be opened if needed.

Storage

  • Store prazosin at room temperature between 68°F and 77°F (20°C and 25°C).
  • Protect this drug from light and moisture.
  • Don’t store this medication in moist or damp areas, such as bathrooms.

Refills

A prescription for this medication is refillable. You should not need a new prescription for this medication to be refilled. Your doctor will write the number of refills authorized on your prescription.

Travel

When traveling with your medication:

  • Always carry your medication with you. When flying, never put it into a checked bag. Keep it in your carry-on bag.
  • Don’t worry about airport x-ray machines. They can’t hurt your medication.
  • You may need to show airport staff the pharmacy label for your medication. Always carry the original prescription-labeled box with you.
  • Don’t put this medication in your car’s glove compartment or leave it in the car. Be sure to avoid doing this when the weather is very hot or very cold.

Self-management

Your doctor may recommend that you check and record your blood pressure daily.

If your doctor instructs you to monitor and record your blood pressure daily, you’ll need a home blood pressure monitor.

Clinical monitoring

Your doctor will regularly check the following to make sure this medication is working and is safe for you:

  • your blood pressure
  • your heart rate (prazosin can cause a fast heart rate known as tachycardia)

Are there any alternatives?

There are other drugs available to treat your condition. Some may be better suited for you than others. Talk to your doctor about other drug options that may work for you.

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up-to-date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or other healthcare professional before taking any medication. The drug information contained here in is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.

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