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Flomax: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on March 1, 2023.

1. How it works

  • Flomax is a brand (trade) name for tamsulosin.
  • Flomax (tamsulosin) works on alpha receptors to relax muscles in the neck of the bladder and the prostate. This improves urine flow and reduces the symptoms of benign prostatic hyperplasia (BPH).
  • Flomax belongs to the class of medicines known as alpha-blockers.

2. Upsides

  • Flomax may be used to improve urine flow in men with benign prostatic hyperplasia (BPH), also referred to as an enlarged prostate gland.
  • Flomax is taken just once daily.
  • Taken orally (by mouth)
  • No dosage adjustment is required in people with kidney disease; however, people with end-stage renal disease have not been studied.
  • Flomax is available as a generic under the name tamsulosin.

3. Downsides

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:

  • A runny nose, dizziness, headache, drowsiness, and sexual dysfunction (decreased semen production, abnormal ejaculation).
  • May cause orthostatic hypotension (a drop in blood pressure when going from a sitting to a standing position). This is more likely during Flomax initiation, or when increasing the dose. Vertigo and syncope (temporary loss of consciousness) may also occur.
  • Rare cases of priapism (painful erections more than 4 hours in duration) have been reported.
  • May not be suitable for people undergoing cataract or glaucoma surgery or for those with a sulfa allergy (although reactions have rarely been reported).
  • May interact with several other medicines including ketoconazole and other strong inhibitors of hepatic enzymes CYP3A4 (avoid this combination), erythromycin, paroxetine, terbinafine, cimetidine, warfarin, PDE 5 inhibitors (such as sildenafil, tadalafil, and vardenafil), and other alpha-adrenergic blocking agents.
  • Rarely, may cause a reaction in people with a sulfa allergy.
  • Flomax capsules are not intended to be used to lower blood pressure.

Note: 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. View complete list of side effects

4. Bottom Line

  • Flomax restores urine flow in men with BPH but may cause a drop in blood pressure when going from a sitting to a standing position.

5. Tips

  • Administer at the same time each day, half an hour after a meal.
  • Take exactly as directed by your doctor. Do not increase or decrease the dosage of Flomax without his or her advice.
  • Do not crush, chew, or open Flomax capsules.
  • Flomax may make you feel dizzy, especially when going from lying down or sitting position to standing. Stand up slowly and not in an area where an injury is likely if you fall over. Remove any potential fall hazards (such as loose rugs) from your home. Talk to your doctor if you constantly experience dizziness when standing up.
  • May cause dizziness and affect your ability to drive or operate machinery. Do not drive or operate machinery until you know how Flomax affects you. Alcohol may enhance dizziness.
  • Since BPH and prostate cancer can coexist, talk to your doctor about screening for prostate cancer.
  • Talk to a doctor or pharmacist before you take any other medications with Flomax, including those bought over the counter, because some may not be compatible with Flomax.
  • Talk to your doctor if you experience any worrying side effects from Flomax including a sustained erection of more than a few hours duration.

6. Response and effectiveness

  • Peak concentrations are reached within four to five hours when taken on an empty stomach or within six to seven hours when taken with food. May take up to five days before the full effects of Flomax on urine flow are achieved.
  • Flomax is extensively metabolized by CYP3A4 and CYP2D6 enzymes which means it can potentially interact with several other drugs.

7. Interactions

Medicines that interact with Flomax may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with Flomax. 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 Flomax include:

  • antibiotics such as clarithromycin, erythromycin, or doxycycline
  • antidepressants, such as fluoxetine, paroxetine, St John’s Wort, monoamine oxidase inhibitors, and tricyclic antidepressants (such as amitriptyline, imipramine, and nortriptyline)
  • antifungals, such as itraconazole and ketoconazole
  • antihistamines, such as diphenhydramine
  • beta-blockers such as atenolol or metoprolol
  • cyclosporine
  • CYP3A4 or CYP2D inducers or inhibitors
  • HIV medications such as ritonavir
  • imatinib
  • other alpha-blockers such as prazosin or terazosin
  • some medications used to treat mental illness, such as clozapine or thioridazine
  • some heart medications, such as amiodarone, clonidine, digoxin, diltiazem, propafenone, quinidine, and verapamil
  • NSAIDs, such as diclofenac, ibuprofen, and indomethacin, may decrease the blood pressure-lowering capabilities of Flomax.

Note that this list is not all-inclusive and includes only common medications that may interact with Flomax. You should refer to the prescribing information for Flomax for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Flomax only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: March 1, 2023.