Facts About Flomax (Tamsulosin)

Flomax (tamsulosin hydrochloride) is a drug used to treat an enlarged prostate and chronic prostate inflammation. It is one of the most commonly prescribed drugs in older men and one of the 200 most prescribed drugs overall, according to the Pharmacy Technician Certification Board.

Flomax was granted approval by the Food and Drug Administration (FDA) in 1997. In 2009, the drug manufacturer Boehringer Ingelheim lost its patent for the drug molecule, and, as a result, there are numerous low-cost generics now available.

This article covers how Flomax works, what the dosage is, how long it takes to work, and common side effects.

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How Does Flomax Work?

Flomax belongs to a class of medications known as alpha-adrenergic blockers that work by relaxing both the bladder neck muscles and the muscle of the prostate gland itself. By doing so, men with prostate problems are able to urinate more freely, reduce the need for nighttime urination (nocturia), and improve urinary urgency and frequency.

Flomax is indicated to treat a number of common medical conditions in men, including:

Despite the fact that Flomax can decrease blood pressure, it is not approved for the treatment of hypertension (high blood pressure).

Flomax Dosage

Flomax is available in a 0.4-milligram (mg) capsule. It is taken daily approximately 30 minutes after a meal, ideally at the same time every day.

If you miss a dose, take it as soon as you remember. However, if it is almost time for your next dose, skip the missed dose and proceed as usual. Never take a double dose of Flomax, since this could lead to a steep drop in blood pressure (hypotension).

Many urologists today will prescribe Flomax as part of a dual therapy. A 2019 meta-analysis found that the combination of tamsulosin plus Avodart (dutasteride) provides greater therapeutic effect for BPH. The combination has a higher incidence of sexual side effects, but can significantly reduce risk of BPH-related symptom progression and acute urinary retention relative to tamsulosin monotherapy.

In 2010, a combination drug called Jalyn (dutasteride 0.5 mg/tamsulosin 0.4 mg) was granted approval by the FDA.

How Long Does Flomax Take to Work?

The effects of the drug can usually be felt within the first 48 hours. Complete urinary relief may take up to two to six weeks. If a 0.4-mg dose is unable to provide relief, your healthcare provider may recommend that you double the dose to 0.8 mg once daily. Never increase the dose unless directed to do so by your healthcare provider.

Common Side Effects

While Flomax is considered safe to use over the long term, there are a number of side effects to be aware of. The most common include (by frequency of occurrence):

  • Headaches
  • Dizziness
  • Rhinitis (hay fever)
  • Ejaculation problems (including retrograde ejaculation)
  • Abnormal weakness
  • Chest pain
  • Cough
  • Drowsiness
  • Nausea
  • Decreased libido
  • Blurred vision
  • Rash
  • Priapism (a painful erection lasting four hours or more)

Other more problematic side effects include dizziness, vertigo, postural hypotension (low blood pressure when rising), and fainting. Collectively known as orthostasis, these symptoms tend to occur within the first eight weeks of treatment. Due to its potential effect on blood pressure, people with low blood pressure, mechanical heart problems, or congestive heart failure should be monitored closely while taking Flomax.

Call your healthcare provider if your urination doesn't improve within two weeks or you experience any severe, persistent, or worsening side effects like fainting or priapism.

Interactions and Contraindications

Flomax is contraindicated for use in people with a known allergy to Flomax or any of its components.

Flomax should not be used with certain CYP3A4 inhibitors. These are drugs that block the CYP3A4 enzyme, a substance produced by the liver that can affect the bioavailability (concentration) of Flomax in the blood. Taking a strong CYP3A4 inhibitor may increase the concentration of Flomax and the severity of side effects (most typically low blood pressure).

In some cases, your blood pressure may need to be monitored while on treatment. In others, one of the two drugs may need to be substituted.

Speak with your healthcare provider if you take any of the following:

Flomax should not be used in people with end-stage kidney failure. People with kidney impairment should also be regularly monitored due to increased risk of an exaggerated hypotensive response.

If you are scheduled to have cataract or glaucoma surgery, tell your healthcare provider if you are either using or have recently used Flomax. A serious eye disorder called intraoperative floppy iris syndrome has been known to occur in around 2% of those who have undergone cataract surgery while on Flomax.

As such, Flomax is not recommended for use prior to cataract surgery due to studies showing an increased risk of a detached retina or lost lens.


Flomax is not indicated for use in females. Due to its potential impact on blood pressure, it is important to advise your healthcare provider if you have dizziness while taking Flomax. Take care when rising from bed in the morning to avoid falling or blacking out. The same applies if you are planning to drive given the risk of vision impairment and dizziness. In some cases, treatment may need to be stopped if the drug interferes with your ability to function normally.

Because BPH closely resembles prostate cancer, people with prostate problems should undergo tests to rule out cancer before starting treatment and at regular intervals afterward.

Generics and Cost

Following the end of Boehringer Ingelheim's patent exclusivity in 2009, the FDA approved the first Flomax generic in March 2010. Today, there are 11 tamsulosin 0.4-mg generics available in the United States, ranging in price from $10 to $30 for a 30-day supply.

For its part, Boehringer Ingelheim offers the brand name Flomax at an online price of $35 for 30 capsules or $90 for 90 capsules with free home delivery.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Flomax. Highlights of prescribing information.

  2. Zhou Z, Cui Y, Wu J, Ding R, Cai T, Gao Z. Meta-analysis of the efficacy and safety of combination of tamsulosin plus dutasteride compared with tamsulosin monotherapy in treating benign prostatic hyperplasia. BMC Urol. 2019;19(1):17. doi:10.1186/s12894-019-0446-8

  3. Food and Drug Administration. JALYN (dutasteride and tamsulosin hydrochloride) capsules.

Additional Reading

By Michael Bihari, MD
Michael Bihari, MD, is a board-certified pediatrician, health educator, and medical writer, and president emeritus of the Community Health Center of Cape Cod.