Key takeaways

Prazosin is a generic drug primarily used to treat high blood pressure. Some healthcare providers prescribe it off-label (for a non-FDA-approved use) for conditions like PTSD-induced nightmares, urinary blockage due to an enlarged prostate, and Raynaud’s phenomenon.
Common side effects of prazosin include dizziness, headache, drowsiness, and weakness, while serious side effects may involve significant drops in blood pressure, pancreatitis, liver abnormalities, and severe allergic reactions.
To minimize or avoid prazosin side effects, patients are advised to follow their prescriber’s instructions carefully, avoid alcohol, stand up slowly to avoid dizziness, and use caution with over-the-counter remedies that may counteract prazosin’s effects.
If side effects occur, such as feeling dizzy or faint, experiencing a headache, fast heartbeats, painful or prolonged erections, or signs of an allergic reaction, patients should seek immediate medical attention or consult with their healthcare provider for appropriate management.

Prazosin is a generic prescription drug that is FDA approved to treat high blood pressure (hypertension). If your blood pressure remains elevated, you’re at greater risk of experiencing a heart attack or stroke. Some healthcare providers prescribe prazosin off-label to help with urinary blockage due to an enlarged prostate, treat congestive heart failure, relieve nightmares in people with post-traumatic stress disorder (PTSD), prevent Raynaud’s phenomenon, relieve “fight-or-flight” symptoms in people with adrenal gland tumors, and, in some countries, to counter scorpion venom.

Its many uses are due to its effects as a selective alpha-1 receptor antagonist or alpha blocker for short. These drugs block the ability of the nervous system to tighten certain types of muscles found in the bladder and blood vessels. As a result, these muscles relax, lowering blood pressure in people with hypertension, allowing more blood flow in people with Raynaud’s, and, for people with enlarged prostate, allowing urine to flow more freely out of the bladder. Also prescribed as the brand name Minipress manufactured by Pfizer, prazosin side effects are common but tend to be mild.

Common side effects of prazosin

The most common side effects of prazosin are mild to moderate in nature. Many are due to the drug’s effects on the nervous system such as dizziness, drowsiness, and fatigue, its most common side effects.

The most common side effects of prazosin are:

Dizziness
Headache
Drowsiness
Lack of energy
Weakness
Heart palpitations
Nausea
Depression
Constipation
Dry mouth
Edema
Blurred vision

Serious side effects of prazosin

The most serious side effects of prazosin are:

Drop in blood pressure after the first dose
Drop in blood pressure when standing up (orthostatic hypotension)
Fainting (syncope)
Pancreatitis
Liver abnormalities
Slowed heart rate
Persistent or painful erections (priapism)
Severe allergic reactions

Prazosin and dizziness

People taking prazosin can expect to have problems with dizziness or related problems. This is because alpha-blockers like prazosin affect the part of the nervous system that’s responsible for all the involuntary activities involved with stress and strenuous physical activity. This part of the nervous system, called the sympathetic nervous system, can be thought of as the “fight or flight” nervous system. It’s the part of the nervous system that comes into play when people are stressed, anxious, scared, or exercising.

It shouldn’t be surprising, then, that prazosin primarily makes people sleepy, dizzy, or just generally tired. In clinical trials, dizziness affected over 10% of participants, while almost 8% complained of sleepiness, 7% of low energy, and another 7% of weakness. These are mostly mild symptoms that may improve over time as the body adapts to the drug. But if dizziness or drowsiness seriously interfere with daily life, talk to the prescriber about other treatment options.

Prazosin and the first dose effect

Prazosin and other alpha blockers may cause dramatic side effects right after the first dose: orthostatic hypotension, dizziness, fainting, sudden loss of consciousness, palpitations (tachycardia), lightheadedness, and lack of energy. Fainting or loss of consciousness can occur within 30 to 90 minutes after the first dose has been taken.

These problems can also happen when the dose is increased, again, right after the first dose. About 1 in 100 people will have fainting issues after the first dose if the dose is 2 mg or higher. Healthcare providers, then, are advised to start prazosin at a low dose—1 mg or even 0.5 mg—and then gradually increase the dose. To avoid injuries due to first-dose dizziness or fainting, the first dose may be taken at bedtime.

Prazosin and hallucinations

Hallucinations are a rare side effect of prazosin, affecting less than one percent of people taking the drug. Prazosin does affect the brain, so side effects like depression, anxiety, and even hallucinations are possible. If hallucinations occur, immediately report this side effect to the prescriber. Prazosin may need to be discontinued.

How soon do prazosin side effects start?

Some prazosin side effects can start within half an hour of the first dose including postural low blood pressure, dizziness, fainting, and mild allergic reactions. Most other common side effects such as headache, drowsiness, weakness, low energy, and nausea can occur early in treatment. Some side effects such as depression or severe side effects are typically delayed, requiring several weeks or months of treatment before becoming a problem.

How long do prazosin side effects last?

Many prazosin side effects will improve over time as the body adapts to the drug. If they don’t, most will go away when the drug is discontinued. With a half-life between two and three hours, prazosin is rapidly cleared from the body. So most mild side effects will dissipate even in the case of a missed dose. Side effects, however, are not a good reason to skip a dose. Talk to the prescriber if side effects are hard to take.

What are the long-term side effects of prazosin?

Prazosin is not associated with side effects that occur due to long-term use. In fact, prazosin is typically prescribed for months or years to control high blood pressure. Side effects with long-term complications are rare for prazosin.

Prazosin contraindications

Prazosin is never prescribed to people who have had a severe allergic reaction to the drug or to drugs that are chemically similar to prazosin called quinazolines.

Pregnancy

Prazosin is used cautiously in pregnant women. Small studies suggest it is safe, but it’s only recommended if the benefits of prazosin outweigh the potential risks to the fetus.

Breastfeeding

Because prazosin is present in human breast milk, healthcare professionals are cautious about prescribing it to mothers who are breastfeeding. They may use alternative treatments or advise the woman not to breastfeed while taking the drug.

Children

The use of prazosin has not been clinically established to be safe and effective in children and adolescents.

Seniors

Older adults can take prazosin, but healthcare providers try to avoid it as a long-term treatment. People older than 65 are more likely to experience orthostatic hypotension, a rapid drop in blood pressure when standing up, and fainting. Both put them at risk for falling, accidents, and injury, so prazosin is considered a potentially inappropriate medication for the elderly. Prazosin may also worsen urinary incontinence, a common problem among people older than 65.

Prazosin warnings

As a drug that affects the nervous system, prazosin has some warnings and cautions people should be aware of.

Bold-faced warning

Prazosin does not have a black box warning—an FDA-mandated warning about serious safety issues—in its drug information. It does, however, have a bold-faced warning about first dose effects. The first dose of prazosin—or the first time an increased dose is taken—can cause serious problems such as postural low blood pressure, fainting, dizziness, and racing heartbeats. The warning urges prescribers to begin treatment at doses lower than the standard dose.

Cautions

Because of its effects on the nervous system, blood vessels, and urinary tract, prazosin can worsen some pre-existing conditions including:

Low blood pressure
Chest pain (angina)
Orthostatic hypotension (a drop in blood pressure when standing up)
Urinary incontinence

Drugs like prazosin can interfere with pupil dilation in people requiring cataract surgery, a side effect called floppy iris syndrome. The effect is to make it difficult to perform cataract surgery. Prazosin may need to be discontinued several days before surgery.

It has been reported that prazosin can worsen narcolepsy, but healthcare professionals are uncertain if prazosin is the cause.

Abuse and dependence

Prazosin is not associated with abuse or physical dependence. While there are no significant withdrawal effects, doses may need to be tapered down in elderly patients.

Overdose

Get immediate medical attention or call a poison control center if too much prazosin is taken. An overdose typically has side effects such as extreme drowsiness and slowed reflexes, but severely low blood pressure is a potential serious problem.

Prazosin interactions

Some classes of drugs can cause problems with prazosin. Some drug interactions add to the adverse effects of prazosin and some work against the beneficial effects of prazosin. These include:

Marplan (isocarboxazid)

This antidepressant is never prescribed to people taking blood pressure medications (antihypertensives).

Other alpha blockers

Taking more than one drug that works like prazosin (called selective alpha-1 receptor antagonists) is not recommended because of the increased risk of side effects. Different types of alpha blockers can cause additive problems, too.

Erectile dysfunction drugs

Taking prazosin with ED drugs like Viagra (sildenafil) and Cialis (tadalafil) could increase the risk of severely low blood pressure.

Sympathomimetics

These adrenaline-like drugs, which include epinephrine and over-the-counter pseudoephedrine, do the exact opposite of prazosin: they activate the “fight-or-flight” nervous system (called the sympathetic nervous system) and so block the effects of prazosin.

Beta blockers

Orthostatic hypotension is more likely when beta blockers like propranolol are combined with prazosin.

Alcohol

Low blood pressure is a risk in people who take prazosin and drink alcohol.

How to avoid prazosin side effects

While prazosin side effects are relatively common, they tend to be mild. Even so, a few tips can help people avoid or minimize problems when taking this drug.

1. Tell the prescriber about all medical conditions and medications

The first rule of thumb to avoid side effects—particularly bad ones—is to make sure the prescriber knows about:

Low blood pressure
Upcoming eye surgery
Pregnancy or pregnancy plans
Breastfeeding or breastfeeding plans
A complete list of all prescription drugs, over-the-counter medications, and dietary supplements being taken

2. Follow the prescriber’s directions

Follow the prescriber’s directions. Take the dose as prescribed at the times prescribed. The prescriber may direct that the first dose be taken at bedtime. Store the medication in a tightly closed container at room temperature, away from heat and moisture. Be sure to keep it out of the reach of children.

3. Keep track of when doses are taken

Prazosin will need to be taken two or three times a day. With a dosing schedule that heavy, it’s easy to forget a dose or forget that a dose has already been taken. To get the maximum benefit of prazosin and avoid side effects, use an alarm or schedule to remember doses. Keep track of doses so that a dose isn’t accidentally taken twice.

4. Be careful about missed doses

Missed doses are another possible pitfall. While a missed dose can be taken when remembered, it shouldn’t be taken when it’s almost time for the next dose. The next dose can arrive quickly when on a three-times-a-day dosing schedule. In those cases, skip the missed dose and take the next dose on time. Never take two doses to make up for a missed dose.

5. Stand up slowly and move carefully

Prazosin can make some people feel woozy or dizzy when standing up. This can be prevented by sitting or standing up slowly.

6. Avoid driving after the first dose

Prazosin can cause drowsiness, dizziness, and even fainting. These are more likely to happen right after the first dose. Avoid driving or other risky activities on the first day of taking prazosin.

7. Avoid drinking alcohol

Alcohol can cause blood pressure to dip too low when taking prazosin. It can also worsen side effects such as sleepiness, dizziness, and vertigo.

8. Avoid some over-the-counter remedies

Anything containing pseudoephedrine will work against prazosin. While it has nothing to do with side effects, it will reduce the benefits of taking prazosin.

How to treat side effects of prazosin

Most people can handle prazosin side effects at home. In rare cases, however, medical attention is needed.

Feeling dizzy or faint

Immediately sit down to avoid falling, passing out, or getting injured. Lying down isn’t advised because it can make the dizziness worse. If driving, pull over and park. When the dizziness or sleepiness gets better, then carry on with what you were doing. Make sure to tell the prescriber that prazosin is causing dizziness or any related problem.

Headache

Drinking fluids and resting can generally help. As far as over-the-counter pain relievers, get medical advice from the prescribing healthcare provider. The prescriber may suggest avoiding NSAIDs like aspirin or ibuprofen because of possible kidney problems. If headaches are bad or don’t go away, talk to the prescriber.

Fast heartbeats

Racing heartbeats or chest pain should be immediately reported to the prescribing healthcare provider.

Painful or prolonged erections

Some men taking prazosin may experience painful or long-lasting erections. If an erection hurts or it lasts for longer than four hours, get immediate medical attention to prevent permanent damage to the penis.

Allergic reactions

Stop taking prazosin and get immediate medical attention at any sign of an allergic reaction such as trouble breathing, hives, and any swelling of the face, lips, mouth, or throat.

Prazosin side effects and how to avoid them