How does this medication work? What will it do for me?

Prazosin belongs to the family of medications called antihypertensives, specifically the group known as alpha 1-blockers. It is used to treat high blood pressure. It relaxes blood vessels, allowing blood to flow through them more easily.

Prazosin may be used alone or in addition to other medications to treat high blood pressure. Prazosin may start working within 1 day, but it may take up to 14 days.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

What form(s) does this medication come in?

Nu-Prazo is no longer being manufactured for sale in Canada. For brands that may still be available, search under prazosin. This article is being kept available for reference purposes only. If you are using this medication, speak with your doctor or pharmacist for information about your treatment options.

How should I use this medication?

The recommended adult dose of prazosin starts with 0.5 mg with the evening meal and is increased gradually until the desired effect is achieved. The maximum dose is 20 mg daily, taken in 2 or 3 divided doses with food. Prazosin may be used alone or in combination with other medications for lowering blood pressure.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not dispose of medications in wastewater (e.g. down the sink or in the toilet) or in household garbage. Ask your pharmacist how to dispose of medications that are no longer needed or have expired.

Who should NOT take this medication?

Do not take this mediction if you are allergic to prazosin or any ingredients of the medication.

What side effects are possible with this medication?

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

  • abdominal discomfort
  • constipation
  • diarrhea
  • dizziness or lightheadedness, especially when rising from a lying or sitting position
  • drowsiness
  • dry mouth
  • headache
  • lack of energy
  • nausea
  • nervousness
  • unusual tiredness or weakness
  • vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

  • blurred vision
  • chest pain
  • fainting (sudden)
  • frequent urge to urinate
  • loss of bladder control
  • painful inappropriate erection of penis (continuing)
  • pounding or irregular heartbeat
  • shortness of breath
  • swelling of feet or lower legs

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Are there any other precautions or warnings for this medication?

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Drowsiness/reduced alertness: This medication can cause severe dizziness and lightheadedness. Avoid driving or operating dangerous machinery until you determine how this medication affects you. Speak with your doctor if you experience severe dizziness or lightheadedness while taking prazosin.

Kidney problems: If you have reduced kidney function, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Orthostatic hypotension: Prazosin has been known to cause severe dizziness and even loss of consciousness for people moving from a sitting or lying position to a standing position. This is known as orthostatic hypotension. It happens most commonly if the person is already taking another blood pressure-lowering medication, has just started treatment with prazosin, or starts treatment with prazosin at a higher dose.

To help prevent this, get up slowly from a sitting or lying position. Your doctor will also recommend slowly increasing the dose of prazosin.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: This medication passes into breast milk. If you are a breast-feeding mother and are taking prazosin, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of using this medication have not been established for children under 12 years of age.

What other drugs could interact with this medication?

There may be an interaction between prazosin and any of the following:

  • dabigatran
  • diuretics (e.g., furosemide, hydrochlorothiazide)
  • linagliptide
  • other alpha 1 blockers (e.g., doxazosin, tamsulosin, terazosin)
  • other medications that reduce blood pressure (e.g., beta-blockers such as metoprolol and propranolol)
  • rituximab
  • sildenafil
  • tadalafil
  • vardenafil

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

  • stop taking one of the medications,
  • change one of the medications to another,
  • change how you are taking one or both of the medications, or
  • leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.