Doxazosin: 7 things you should know
Medically reviewed by Carmen Fookes, BPharm. Last updated on Jun 10, 2020.
1. How it works
- Doxazosin blocks alpha-1 receptors. Stimulation of alpha-1 receptors is associated with a tightening up of the smooth muscle located in the prostate and neck of the bladder. This muscle tightening can detrimentally affect urine flow and frequency, and cause symptoms such as those seen in Benign Prostatic Hyperplasia (BPH). Doxazosin blocks these receptors allowing this muscle to relax.
- Doxazosin also lowers blood pressure by selectively blocking alpha-1 receptors located in the smooth muscle lining blood vessels, allowing the blood vessels to widen (dilate). Doxazosin may also block the nerve receptors responsible for the contraction (narrowing) of the blood vessels.
- Doxazosin belongs to the group of medicines known as alpha-1 adrenergic blockers (often shortened to alpha-blockers).
- Doxazosin is used in the treatment of BPH and high blood pressure.
- Trials have shown that lowering blood pressure reduces the risk of a stroke, myocardial infarction (heart attack) and other cardiovascular events.
- Doxazosin's effects on blood pressure are more significant when standing.
- Does not adversely affect blood lipid (cholesterol) levels.
- Can be used alone or in combination with other antihypertensive agents such as diuretics and beta blockers.
- Side effects are usually mild and short-lived.
- Available as a tablet and an extended-release tablet. The extended-release tablet is only used to treat BPH (not high blood pressure).
- Usually taken once a day.
- Generic doxazosin is available.
If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:
- Dizziness or drowsiness which may affect your ability to drive or operate machinery, particularly within the first 24 hours of dosing, when the dose is increased, when going from a lying down to standing position, during hot weather, after exercise, or after drinking alcohol. Do not drive or operate machinery until you know how doxazosin affects you. Getting up slowly may help lessen the problem.
- Low blood pressure, shortness of breath, dry mouth, edema, rhinitis and fatigue may also occur.
- May rarely cause syncope (fainting - or a temporary loss of consciousness), especially when going from a lying or sitting position to a standing position. These episodes may occur within 30 to 90 minutes of taking doxazosin and the risk of an episode is higher during a dosage increase or when doxazosin is used in combination with another antihypertensive drug.
- Sexual dysfunction is uncommon. Rarely, may cause prolonged erections lasting more than four hours. Seek immediate medical help.
- May not be suitable for some people including those undergoing cataract surgery, with low blood pressure, intestinal blockages, severe constipation, or liver disease. Because prostate cancer causes many of the same symptoms of BPH, it should be ruled out before starting doxazosin.
- Doxazosin may interact with some drugs, including phosphodiesterase-5 (PDE-5) inhibitors (such as sildenafil, vardenafil), drugs that inhibit CYP 3A4 hepatic enzymes, and those that also lower blood pressure or cause dizziness. Should not be taken by people who are allergic to doxazosin or related drugs such as alfuzosin, prazosin, silodosin, tamsulosin, or terazosin.
Notes: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. For a complete list of all side effects, click here.
- Doxazosin is usually given once daily.
- If you feel yourself beginning to faint or feeling dizzy or lightheaded while on doxazosin, lie down until the feeling passes. This effect is usually short-lived and likely only to occur on therapy initiation or during dose titration. Be careful with the amount of alcohol you drink when on doxazosin as alcohol may increase the risk of becoming dizzy or lightheaded. Drink plenty of fluids during the day to maintain good hydration.
- Doxazosin is usually started at a low dosage, which is increased slowly. If you discontinue doxazosin for several days, you may need to start it again at a lower dosage and slowly titrate it up. Seek your doctor's advice.
- Good blood pressure control requires lifestyle changes to also be made, such as stopping smoking, reducing dietary sodium intake, and increasing levels of physical activity. In addition, blood sugars and lipids should be kept within normal limits and antithrombotic therapy should be considered for certain people.
- Many patients require more than one drug to achieve blood pressure targets.
- Doxazosin may interact with drugs used for erectile dysfunction (ED). Talk to your doctor first before taking any drugs for ED.
- If you need to have cataract surgery, tell your healthcare provider that you take doxazosin tablets.
6. Response and Effectiveness
- Blood levels of doxazosin reach a peak within two to three hours of an oral dose.
- The blood pressure lowering effect occurs within one to six hours of a dose of doxazosin.
- Relief from symptoms of BPH and an increase in urine flow have been reported as early as one week after the initiation of doxazosin.
Medicines that interact with doxazosin may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with doxazosin. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.
Common medications that may interact with doxazosin include:
- erectile dysfunction medications (eg, alprostadil, sildenafil, tadalafil, or vardenafil)
- hepatitis C medicines, such as boceprevir
- other heart medications that may also lower blood pressure, such as atenolol, carvedilol, clonidine, diltiazem, labetalol, metoprolol, nadolol, propranolol, sotalol, trandolapril, or verapamil
- medicines that also inhibit the group of hepatic enzymes known as CYP3A4 (such as atazanavir, clarithromycin, darunavir, indinavir, itraconazole, ketoconazole, lopinavir, nefazodone, nelfinavir, ritonavir, saquinavir, telithromycin, or tipranavir
- monamine oxidase inhibitor antidepressants, such as phenelzine or tranylcypromine.
Note that this list is not all-inclusive and includes only common medications that may interact with doxazosin. You should refer to the prescribing information for doxazosin for a complete list of interactions.
Doxazosin. Revised 12/2019. Drugs.com https://www.drugs.com/ppa/doxazosin.html
Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use doxazosin only for the indication prescribed.
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- Drug class: antiadrenergic agents, peripherally acting