Prazosin dosage, forms, and strengths

Prazosin is the name of a generic prescription high blood pressure medication. Healthcare professionals sometimes use it off-label to treat enlarged prostate, Raynaud’s syndrome, and post-traumatic stress disorder (PTSD). Prazosin lowers blood pressure by relaxing the smooth muscles of blood vessels. To control blood pressure, prazosin capsules are taken three times a day. 

Prazosin forms and strengths

• Capsules: 1 mg, 2 mg, 5 mg

Prazosin dosage for adults

The U.S. Food and Drug Administration (FDA) has approved prazosin as an adult high blood pressure treatment. Still, healthcare providers may use it off-label for other conditions, including benign prostatic hyperplasia (BPH), but it is not a recommended treatment for BPH. The only adults who cannot safely take prazosin are those with a history of allergic reactions to it or quinazoline drugs like it.

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Prazosin dosage for hypertension

Prazosin and similar drugs are not first-choice treatments for high blood pressure. Prazosin is an alpha-1 receptor blocker that blocks the ability of nerves to tighten smooth muscles in peripheral blood vessels. By relaxing the most distant and smallest arteries and veins in the body, prazosin lowers resistance in the circulatory system, reducing blood pressure. 

Alpha blockers are usually prescribed after people have used other blood pressure drugs and still have not achieved normal blood pressure. When these other drugs fail or stop working, healthcare providers often add drugs like prazosin to the regimen. Prazosin can be used alone to treat blood pressure, but healthcare providers often prescribe it along with other medications.

Doses start small—just one milligram two or three times a day. These dosages can rise to as high as 20 mg daily but tend to range between 6 mg to 15 mg daily. In extreme cases, a healthcare provider can prescribe dosages as high as 40 mg per day. The patient is often asked to take their blood pressure daily. It is best to take blood pressure at different times of day, after resting for 5 minutes, and have the blood pressure cuff at the elevation of the heart. Record blood pressure in a blood pressure log. Bring this log to review with your provider to assess if medication dosages are adequate. 

The very first dose may cause drowsiness, dizziness, or other impairment. Don’t be surprised if a healthcare provider advises that the first dose be taken at bedtime. In any case, people taking prazosin for the very first time should avoid driving or other risky activities after they take the first couple of doses.

• Standard prazosin dosage for hypertension as sole therapy: 6–20 mg daily taken in divided doses

• Standard prazosin dosage for hypertension with other antihypertensives or diuretics: 1–2 mg taken two or three times daily

• Maximum prazosin dosage for hypertension: 40 mg daily taken in divided doses

Prazosin dosage for children

The FDA has not approved the use of prazosin in children.

Prazosin dosage restrictions

Prazosin requires no dosage modifications because of age, kidney disease, or liver problems (hepatic impairment).

Prazosin dosage for pets

Although not a veterinarian’s first choice, prazosin can be used in cats and dogs to treat heart failure, high blood pressure, or pulmonary hypertension or to relax the urinary bladder sphincter or the urethra to make it easier for the animal to urinate. Unlike other alpha-1 blockers, prazosin is less likely to cause tachycardia (racing heartbeats) in a pet. 

The dosage will vary based on the animal, its body weight, and the condition being treated. Standard dosages for dogs range from 1–4 mg per day, taken in one to three doses. Cats are usually given 0.25–0.5 mg daily as a single dose or divided into two or three daily doses. Pet parents may have to use human capsules, so dosing may be difficult.

How to take prazosin

Because of the medication’s short half-life, prazosin has to be taken two or three times per day to be effective. Many people with high blood pressure are more accustomed to taking single daily doses of other antihypertensive agents. An alarm or other reminder may help people keep up with frequent prazosin doses. Otherwise, taking prazosin is as basic as taking other blood pressure drugs:

• Take prazosin capsules as directed. The dose may need to be changed to find what works best.

• Prazosin can be taken with or without food. 

• Swallow the capsule whole with a glass of water.

• Store prazosin capsules at room temperature in a securely closed, light-resistant container.

Prazosin dosage FAQs

How long does prazosin take to work?

Prazosin relaxes peripheral blood vessels from the very first dose. A prazosin dose reaches its peak effects on blood pressure two to four hours after a capsule is swallowed. However, it may take four to six weeks for prazosin therapy to achieve its full effects in lowering blood pressure.

How long does prazosin stay in your system?

With a half-life of two to four hours, prazosin is cleared from the body in 10 to 20 hours.

What happens if I miss a dose of prazosin?

If a prazosin dose is missed or forgotten, take the missed dose if it’s not almost time for the next dose. If the next dose is coming up, skip the missed dose and take the next dose as planned. Never take extra medicine to make up for a missed dose, as this may cause hypotension, dizziness, and a fall. 

How long can you take prazosin?

Like most blood pressure treatments, prazosin is intended for long-term therapy that lasts months or years. 

How do I stop taking prazosin?

Prazosin can be stopped at any time without causing withdrawal. However, do not stop taking prazosin until talking to a healthcare provider. If an appropriate blood pressure drug is needed but not taken, a rebound in high blood pressure can result. 

Healthcare providers will stop using prazosin in anyone who has a hypersensitivity reaction to the drug or any worrisome adverse effects. Fortunately, there are various other blood pressure drugs, including ACE inhibitors, angiotensin-II receptor blockers, beta blockers, calcium channel blockers, diuretics, and other alpha blockers.

What is the maximum dosage for prazosin?

The highest recommended dosage for prazosin is 20 mg daily for hypertension, but the FDA has approved doses as high as 40 mg daily if necessary.

Can you overdose on prazosin?

A prazosin overdose usually causes drowsiness, but there is the potential threat of a severe drop in blood pressure. Do not take extra prazosin capsules for any reason. If you do, get medical help or call a poison control center. Get immediate medical help if there are symptoms of severe hypotension, such as dizziness, lightheadedness, blacking out when standing up (orthostatic hypotension), fainting, confusion, fatigue, vision problems, or loss of consciousness.

What interacts with prazosin?

Prazosin has few major drug interactions. Even so, make sure the prescribing healthcare provider knows about all the prescription drugs, over-the-counter medications, and supplements being taken, just in case.

Prazosin’s most significant drug interactions are with drugs that have the opposite effect. Prazosin blocks the nerves that tighten blood vessels. Drugs called sympathomimetics, like epinephrine, norepinephrine, or phenylephrine, do the opposite, neutralizing the benefits of prazosin in lowering blood pressure. 

Healthcare providers are also unlikely to prescribe prazosin in combination with other drugs in the same class (alpha blockers) as the combination will compound the drug's effects on blood pressure.

Combining prazosin with erectile dysfunction drugs called phosphodiesterase 5 (PDE 5) inhibitors like Viagra (sildenafil) could cause blood pressure to drop too low.

What happens when you mix prazosin and alcohol?

Do not drink alcohol when taking prazosin. Alcohol also reduces blood pressure and can worsen side effects like drowsiness and dizziness.

Is it safe to take prazosin during pregnancy?

Healthcare providers use prazosin in pregnant women. There is no evidence that it causes birth defects or affects pregnancy. 

Is it safe to take prazosin during breastfeeding?

Healthcare professionals are cautious about using prazosin in breastfeeding women. Prazosin is present in breast milk, and it’s unknown if it’s safe for nursing infants, particularly newborns or preterm babies. Nursing mothers should ask the prescribing healthcare provider for medical advice and possible alternatives.

What are the side effects of prazosin?

The most common side effects of prazosin are dizziness, headache, drowsiness, lack of energy, weakness, palpitations, dry mouth, and fainting (syncope). Less common but more worrisome side effects include orthostatic hypotension, constipation, blurred vision, low blood pressure, fluid retention (edema), and painful or long-lasting erections (priapism).

What are the serious side effects of prazosin?

Serious adverse effects of prazosin include slow heart rate, swollen pancreas, and low blood pressure. Prazosin can also cause a condition called intraoperative floppy iris syndrome (IFIS), which can cause complications in people undergoing cataract surgery.

Sources

• Medical treatment of benign prostatic hyperplasia, American Family Physician

• Plumb’s Veterinary Drug Manual, 7th ed

• Prazosin, Drugs, and Lactation Database

• Prazosin, StatPearls

• Prazosin, Veterinary Partner

• Prazosin capsule prescribing information, DailyMed (NIH National Library of Medicine)

• Minipress (prazosin hydrochloride) drug summary, Prescriber’s Digital Reference (PDR)