Tamsulosin (Flomax) interactions: what you need to know

Tamsulosin (brand name Flomax) is a medication used to treat benign prostatic hyperplasia (BPH). As with many medications, potential drug interactions may occur with tamsulosin. It's important to tell your healthcare provider about all medications (both prescription and nonprescription), supplements, and herbal products you're taking before beginning a new medication.

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Tamsulosin (brand name Flomax) is an alpha-blocker that's FDA-approved to treat the urinary symptoms of BPH, also referred to as an enlarged prostate. You can only get it with a prescription, but it is available as either generic tamsulosin or brand name Flomax.

Sildenafil and other PDE5 inhibitors

Phosphodiesterase-5 (PDE5) inhibitors are oral medications used to treat erectile dysfunction (ED). They work by dilating blood vessels to allow more blood flow into the penis, improving your erection. However, dilating blood vessels also lowers blood pressure. Taking both tamsulosin and PDE5 inhibitors may low blood pressure, also called hypotension (DailyMed, 2017). Examples of PDE5 inhibitors include sildenafil (brand name Viagra), tadalafil (brand name Cialis), vardenafil (brand name Levitra), and avanafil (brand name Stendra). Ask your healthcare provider if it's safe to take a PDE5 inhibitor while taking tamsulosin.

CYP3A4 and CYP2D6 blockers 

CYP3A4 and CYP2D6 are two enzymes in the liver that break down tamsulosin. Any drug that blocks or inhibits CYP3A4 and CYP2D6 can increase the concentration of tamsulosin in the body. This can increase the likelihood of developing adverse effects, like hypotension and syncope (fainting) (DailyMed, 2017). In some cases, your healthcare provider may adjust your medication dose. Examples of these drugs include cimetidine, ketoconazole, erythromycin, terbinafine, and paroxetine.

Blood pressure medications

Since tamsulosin may lower your blood pressure, you should be careful while being treated for high blood pressure. If you're already taking medication for high blood pressure (hypertension), ask your healthcare provider if it's safe for you to take tamsulosin as well. Taking tamsulosin along with blood pressure medications can increase the risk of orthostatic hypotension (OH), which is dizziness or fainting upon standing; these symptoms are due to a sudden drop in blood pressure (Biaggioni, 2018).

Tamsulosin is FDA-approved to relieve symptoms of benign prostatic hyperplasia (BPH), otherwise known as an enlarged prostate (DailyMed, 2017). As men get older, their prostates naturally get larger. In some men, this can put pressure on the urethra and bladder, partially obstructing the flow of urine. This can cause symptoms like:

• Increased need to urinate, particularly at night

• Difficulty urinating (straining, frequent starting and stopping)

• Weak urine stream

• Trouble beginning urination

Tamsulosin causes smooth muscles in the prostate and bladder to relax, making urination easier.

Tamsulosin is also prescribed "off-label" for chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) and kidney stones that have moved into the urethra (UpToDate, n.d.). By causing smooth muscles in the area to relax, tamsulosin can help dislodge those stones, allowing them to be passed naturally in the urine stream.

Tamsulosin (or tamsulosin hydrochloride) is part of a class of drugs called alpha blockers, also called selective alpha-1a adrenergic receptor antagonists (NIDDK, 2018). Tamsulosin targets receptors in the smooth muscles of the prostate and bladder, causing them to relax, thereby improving symptoms.

Other examples of alpha blockers include alfuzosin (brand name Uroxatral), doxazosin (brand name Cardura), silodosin (brand name Rapaflo), and terazosin (brand name Hytrin).

Like many medications, tamsulosin can cause side effects. These can range from mild to serious.

Common side effects of tamsulosin include:

• Headache

• Dizziness 

• Abnormal ejaculation

• Runny nose or nasal congestion

• Drowsiness

• Back pain

• Stomach upset, diarrhea, and nausea 

Less common side effects of tamsulosin include 

• Orthostatic hypotension, which is dizziness, lightheadedness, or fainting (syncope) when moving from a sitting to standing position. It's caused by a sudden drop in blood pressure.

• Chest pain

• Loss of libido

• Intraoperative floppy iris syndrome (IFIS) which can occur during glaucoma surgery or cataract surgery. 

• Priapism, a painful erection that lasts more than four hours

Some people who are allergic to sulfonamides, or sulfa drugs, may also have an allergic reaction to tamsulosin. Allergic reactions may include a skin rash, swelling, or trouble breathing. 

This is not a complete list of side effects. Seek medical advice from your healthcare professional or pharmacist if you have questions about tamsulosin side effects or drug interactions.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

November 11, 2020

Written by

Michael Martin

Fact checked by

Chimene Richa, MD

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About the medical reviewer

Chimene Richa, MD

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.