Drug Information

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

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

Terazosin is also used to treat enlarged prostate (benign prostatic hyperplasia or BPH). For BPH, terazosin helps to relax the muscles in the prostate and bladder. This helps to improve urine flow and decrease symptoms of BPH. Terazosin does not slow or stop the progression of enlarged prostate.

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.

How should I use this medication?

The recommended adult dose of terazosin for high blood pressure starts at 1 mg at bedtime and is slowly increased until the desired blood pressure response is achieved. The usual dose ranges from 1 mg to 5 mg daily. The dose for high blood pressure is usually taken once or twice a day.

The recommended adult dose of terazosin to treat an enlarged prostate starts at 1 mg at bedtime, with increases at weekly intervals to 2 mg, 5 mg, or 10 mg once daily to achieve the desired improvement of symptoms. It may take 2 to 4 weeks of treatment before you see an improvement of your symptoms.

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.

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 it has been more than a few days since your last dose, talk to your doctor; you may need to restart at the 1 mg dose. 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.

What form(s) does this medication come in?

1 mg
Each white, flat face, bevelled-edged, round tablet debossed "TRZ" on one side and "1" on the other contains 1 mg terazosin hydrochloride. Nonmedicinal ingredients: corn starch, crospovidone, lactose, magnesium stearate, povidone and talc

2 mg
Each orange, flat face, bevelled-edged, round tablet debossed "TRZ" on one side and "2" on the other contains 2 mg of terazosin hydrochloride. Nonmedicinal ingredients: corn starch, crospovidone, lactose, magnesium stearate, povidone and talc, FD&C yellow No. 6 Lake.

5 mg
Each brown-beige, flat face, bevelled-edged, round tablet debossed "TRZ" on one side and "5" on the other contains 5 mg terazosin hydrochloride. Nonmedicinal ingredients: corn starch, crospovidone, lactose, magnesium stearate, povidone and talc, Black iron oxide, iron oxide red and yellow iron oxide.

10 mg
Each blue, flat face, bevelled-edged, round tablet debossed "TRZ" on one side and "10" on the other contains 10 mg of terazosin hydrochloride. Nonmedicinal ingredients: starch, crospovidone, lactose, magnesium stearate, povidone and talc, FD&C Blue No. 1 Lake and FD&C Blue No. 2 Aluminum Lake.

Who should NOT take this medication?

Do not take this medication if you:

  • are allergic to terazosin or any ingredients of this medication
  • are allergic to quinazoline-type medications
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 pain or discomfort
  • back or joint pain
  • blurred vision
  • constipation
  • decreased interest in sexual activity
  • decreased sexual ability
  • diarrhea
  • difficulty sleeping
  • dizziness or lightheadedness, especially when getting up from a lying or sitting position
  • drowsiness
  • dry mouth
  • gas
  • headache
  • increased need to urinate or urinating more often than usual
  • indigestion
  • nausea
  • stuffy nose
  • sweating
  • 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:

  • anxiety
  • blood pressure changes
  • blurred vision
  • chest pain
  • fainting (sudden)
  • fast or irregular heartbeat
  • pounding heartbeat
  • shortness of breath
  • signs of depression (e.g., poor concentration, changes in weight, changes in sleep, decreased interest in activities, thoughts of suicide)
  • skin rash
  • swelling of feet or lower legs
  • symptoms of a urinary tract infection (e.g., pain when urinating, urinating more often than usual, low back or flank pain, strong odour)
  • weight gain

Stop taking the medication and seek immediate medical attention if any of the following occur:

  • painful, prolonged erection (lasting longer than 4 hours)
  • signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)
  • trouble breathing

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.

Blood pressure: The first few doses of terazosin can cause extreme lowering of blood pressure, especially when moving from a lying or sitting position to a standing position. Sometimes even loss of consciousness can occur. A similar effect may occur if the medication is taken again after stopping it for more than a few doses, if the dose is rapidly increased, or if another blood-pressure-lowering medication is added.

The likelihood of this effect can be minimized by starting the dose of terazosin at 1 mg at bedtime, by increasing the dosage slowly, and by starting any additional blood-pressure-lowering medications with caution. If you are also taking phosphodiesterase 5 inhibitors (e.g., sildenafil, tadalafil, vardenafil) discuss with your doctor whether any special monitoring is needed.

While temporary loss of consciousness is the most severe blood-pressure-lowering effect of terazosin, other symptoms of lowered blood pressure, such as dizziness, lightheadedness, and palpitations are more common. If these symptoms occur, lie down and wait for a few minutes before standing to prevent their recurrence, then get up very gradually.

Cataracts: During eye surgery for cataracts, people who take or have taken terazosin are at risk for developing a condition called Intraoperative Floppy Iris Syndrome (IFIS). This condition can lead to eye damage. If you will be undergoing cataract surgery, tell your doctor that you are taking a medication that contains terazosin. Your surgeon may advise you to temporarily stop taking the medication before the surgery.

Disease progression: Terazosin does not slow or stop the progression of enlarged prostate.

Drowsiness/reduced alertness: Because of the risk of extreme lowering of blood pressure, avoid driving or hazardous tasks for 12 hours after the initial dose of terazosin, after the dose is increased, and after interruption of therapy when treatment is resumed.

Avoid situations where dizziness, lightheadedness, or temporary loss of consciousness could result in injury.

Fainting: Terazosin can cause extreme lowering of blood pressure and possibly fainting. If you have a history of micturition syncope (a condition where fainting occurs during or after urination), you should not take terazosin. Talk to your doctor if you have any concerns.

Kidney, bladder, and urinary problems: If you have reduced kidney function, bladder problems, or urinary problems, 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.

Prostate cancer: Prostate cancer and enlarged prostate (BPH) have many of the same symptoms and it is possible for men to have both diseases at the same time. Terazosin is not used to treat prostate cancer. Your doctor may perform tests to rule out prostate cancer before starting you on terazosin treatment.

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: It is not known if terazosin passes into breast milk. If you are a breast-feeding mother and are taking this medication, 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.

Seniors: People over 65 years of age may be at an increased risk of blood pressure decreases, especially when moving from a lying or sitting position to a standing position.

What other drugs could interact with this medication?

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

  • aldesleukin
  • aliskiren
  • other alpha blockers (e.g., alfuzosin, doxazosin, prazosin, tamsulosin)
  • alpha agonists (e.g., clonidine, methyldopa)
  • amiodarone
  • amphetamines (e.g., dextroamphetamine, lisdexamfetamine)
  • angiotensin converting enzyme inhibitors (ACEIs; captopril, enalapril, ramipril)
  • angiotensin receptor blockers (ARBs; e.g., candasartan, irbesartan, losartan)
  • antipsychotics (e.g., aripiprazole, chlorpromazine, lurasidone, quetiapine, risperidone)
  • apomorphine
  • barbiturates (e.g., butalbital, pentobarbital phenobarbital)
  • beta-adrenergic blockers (e.g., atenolol, propranolol, sotalol)
  • brimonidine
  • bromocriptine
  • calcium channel blockers (e.g., amlodipine, diltiazem, nifedipine, verapamil)
  • dexmethylphenidate
  • dipyridamole
  • diuretics (water pills; e.g., furosemide, hydrochlorothiazide, triamterene)
  • duloxetine
  • epinephrine
  • guanfacine
  • levodopa
  • methylphenidate
  • minoxidil
  • monoamine oxidase inhibitors (MAOIs; e.g., moclobemide, phenelzine, rasagiline, selegiline, tranylcypromine)
  • morphine
  • nabilone
  • nasal decongestants (e.g., naphazoline, oxymetazoline, xylometazoline)
  • nitrates (e.g., nitroglycerin, isosorbide dinitrate, isosorbide mononitrate)
  • norepinephrine
  • obinutuzumab
  • pentoxifylline
  • phenylephrine
  • phosphodiesterase 5 inhibitors (e.g., sildenafil, tadalafil, vardenafil)
  • pramipexole
  • pseudoephedrine
  • riociguat
  • rituximab
  • ropinirole
  • sacubitril
  • thioridazine
  • tricyclic antidepressants (e.g., clomipramine, imipramine)
  • yohimbine

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.

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