Tamsulosin (Flomax): everything you need to know

If you are one of the many men who are affected by benign prostatic hyperplasia (BPH), you may have heard of tamsulosin. Tamsulosin hydrochloride (brand name Flomax) is a prescription medication used to treat BPH.

Tamsulosin belongs to a class of medication called alpha blockers that relaxes the muscles of the prostate and bladder. The prostate and bladder have smooth muscles that squeeze (constrict) and relax (dilate) to control the flow of urine. These muscles are controlled by the sympathetic nervous system and the alpha-1 receptors. Stimulating the alpha-1 receptors causes the smooth muscle to constrict while blocking the receptors leads to muscle relaxation. 

Alpha blockers, like tamsulosin, improve urine flow, and decrease the symptoms associated with an enlarged prostate (DailyMed, 2015). Other examples of alpha blockers include alfuzosin (brand name Uroxatral), doxazosin (brand name Cardura), silodosin (brand name Rapaflo), and terazosin (brand name Hytrin).

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Tamsulosin is FDA-approved to treat benign prostatic hyperplasia (BPH). It is not indicated for the treatment of high blood pressure, as is the case with other alpha-blockers like prazosin.

Benign prostatic hyperplasia (BPH)

Benign prostatic hyperplasia, also known as an enlarged prostate, affects approximately half of all men between the ages of 51 and 60 (AUA-a, 2020). As men age, the likelihood of having BPH increases to about 90 percent in men over age 80 (AUA-a, 2020). The prostate gland sits at the base of the bladder and surrounds part of the urethra (the tube that carries urine from the bladder to the penis). 

The primary role of the prostate gland is to make fluid for semen. During ejaculation, fluid from the prostate moves into the urethra and joins sperm from the testicles. This fluid mixture, now called semen, then travels down the urethra and out through the penis.

As the prostate gets larger, it puts pressure on the urethra—this causes common BPH symptoms, like needing to urinate more often, especially at night. Tamsulosin helps to relax the muscles of the prostate and bladder, working quickly to improve the flow of urine, lessen blockage of the urethra, and decrease many of the symptoms caused by the enlarged prostate (DailyMed, 2015). Common symptoms of BPH include lower urinary tract symptoms (LUTS) such as (AUA-a, 2020):

• Urge to urinate frequently

• Urge to urinate more often at night (nocturia)

• Trouble starting urination

• A sensation that your bladder is full, even after you have just finished urinating

• A weak stream of urine

• Frequent starting and stopping during urination

• Having to strain to urinate

Since BPH and prostate cancer often exist together, talk to your healthcare provider about prostate cancer screening before starting treatment and at regular intervals afterward.

Off-label uses

Like many other medications, healthcare providers may use tamsulosin for "off-label" uses. "Off label" means that tamsulosin is not specifically FDA-approved for these uses, which include (UpToDate, n.d.):

• Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS): Chronic prostatitis/chronic pelvic pain syndrome is the most common type of prostatitis and can occur in men at any age—about 90% of men with symptoms from prostatitis have this type (Magistro, 2016). The symptoms of CP/CPPS may come and go and include pelvic pain, pain with ejaculation, urination problems. Additionally, sexual dysfunction, like premature ejaculation, decreased libido, and erectile dysfunction can be an issue for 40–70% of men with CP/CPPS (Magistro, 2016).

• Ureteral stones: Stones that form in the urinary system usually start in the kidneys and are most commonly calcium stones. If a stone passes from the kidney and gets stuck in the ureter, it becomes a ureteral stone. Ureteral stones can block urine flow from that kidney—this can be painful. Symptoms include sharp, cramping back pain that may migrate to the groin. This pain often comes in waves and may be accompanied by blood in the urine. Tamsulosin may benefit by relaxing the ureter, making it easier for the stone to become "unstuck" and pass through your system (AUA-b, 2020).

Common side effects include (DailyMed, 2015):

• Headache

• Dizziness

• "Common cold" symptoms

• Runny nose or nasal congestion

• Abnormal ejaculation

• Diarrhea

• Drowsiness

• Intraoperative floppy iris syndrome (floppy iris during cataract surgery or glaucoma surgery)

Serious side effects include (UpToDate, n.d.):

• Low blood pressures or fainting (syncope), especially when standing from a sitting position (orthostatic hypotension); this is more likely to occur after taking your first dose or an increased dose.

• Chest pain (angina)

• Severe allergic reaction (hypersensitivity) with skin rash, swelling, trouble breathing, etc.

• Priapism: A painful erection that lasts more than four hours

This list may not include all side effects of tamsulosin, and others may occur. Seek medical advice from your healthcare provider or pharmacist for more information.

Talk to your healthcare provider about any other drugs that you are taking before starting tamsulosin, including prescription drugs, over-the-counter medications, and supplements. Potential drug interactions include (DailyMed, 2015):

• Drugs that block the CYP3A4 and CYP2D6 systems: The CYP3A4 and CYP2D6 liver systems break down tamsulosin. Any drug that blocks or inhibits these systems will affect tamsulosin metabolism—this can increase the concentration of tamsulosin in the body and the risk of adverse effects. Examples include cimetidine, ketoconazole, erythromycin, terbinafine, and paroxetine. If you are taking these drugs, your dose of tamsulosin may need adjusting.

• PDE5 inhibitors: Phosphodiesterase-5 (PDE5) inhibitors are commonly used to treat erectile dysfunction; however, they also have the side effect of lowering blood pressure. Taking both tamsulosin and PDE5 inhibitors may cause your blood pressure to drop too low (hypotension). Examples of PDE5 inhibitors include sildenafil (brand name Viagra; see Important Safety Information), tadalafil (brand name Cialis; see Important Safety Information), vardenafil (brand name Levitra), and avanafil (brand name Stendra).

This list may not include all drug interactions with tamsulosin, and others may exist. Seek medical advice from your healthcare provider or pharmacist for more information.

Tamsulosin may increase the likelihood of side effects in certain people—these groups of people should avoid using tamsulosin or use it with caution and careful monitoring. Examples include (UpToDate, n.d.):

• Pregnant or lactating women: Tamsulosin is classified as pregnancy category B, meaning that animal studies have not shown any harm to the fetus. However, tamsulosin is not indicated for use in women.

• People with low blood pressure: If you have low blood pressure or positional blood pressure changes, like orthostatic hypotension, use tamsulosin with caution. It decreases blood pressure and can worsen already low pressures, especially after taking your first dose or an increased dose.

• People who have cataracts or glaucoma: Tamsulosin may affect the iris and make it "floppy" during ocular surgery, especially cataract surgery or glaucoma surgery. This condition is called intraoperative floppy iris syndrome (IFIS). IFIS increases the risk of complications after ocular surgery. IFIS can occur in people taking tamsulosin at the time of surgery, but also in those who stopped the drug weeks to months before ocular surgery. You should not start taking tamsulosin if you are planning to have cataract or glaucoma surgery.

• People with heart failure: Tamsulosin may worsen existing heart failure.

• People with sulfa allergies: Some people who are allergic to drugs in the sulfonamide (sulfa) class have also had allergic reactions to tamsulosin. If you have had a severe allergic reaction to sulfa in the past, use tamsulosin with caution.

This list does not include all at-risk groups, and others may exist. Talk to your healthcare provider or pharmacist for more information.

Tamsulosin hydrochloride is available both as a generic medication and under the brand name Flomax. It is usually taken by mouth once a day and comes in 0.4 mg strength capsules. You should ideally take it about 30 minutes after a meal around the same time each day (MedlinePlus, 2020). Do not crush, chew, or open the capsules. Most prescription plans cover tamsulosin; the cost for a 30-day supply ranges around $9–$35 (GoodRx).

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Current version

September 14, 2020

Written by

Chimene Richa, MD

Fact checked by

Mike Bohl, MD, MPH, ALM

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About the medical reviewer

Mike Bohl, MD, MPH, ALM

Dr. Mike is a licensed physician and a former Director, Medical Content & Education at Ro.