Published Apr. 21, 2023

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PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet.

What Is Tamsulosin?

Tamsulosin is a prescription medication that can be used in dogs and cats with specific conditions that require the urethra and the ureters of the lower urinary tract to relax. Tamsulosin may also help with the passage of small bladder stones, treating functional urinary tract obstruction, benign overgrowth of the prostate, and urethral spasms that occur from a urinary tract obstruction (UTI).

Tamsulosin is FDA-approved for human use under the brand name Flomax® and the generic, tamsulosin. Tamsulosin is currently not FDA-approved as a veterinary medication. However, it is readily utilized in the veterinary field, and veterinarians can legally prescribe certain human drugs in animals in some circumstances. This is called extra-label or off-label use because this use isn’t described on the drug label. Your veterinarian will determine whether this medication is right for your pet.

How Tamsulosin Works

Tamsulosin is categorized as an alpha-1 adrenergic antagonist. Tamsulosin specifically targets the 1A receptors found in the bladder, urethra, and prostate, which are responsible for relaxing the smooth muscles in these areas. Relaxing these muscles aids in the passage of stones and urine.

Tamsulosin Directions

Follow the directions on the drug label or as provided by your veterinarian. Tamsulosin is generally given once daily by mouth in both dogs and cats.

Tamsulosin is available as a capsule that should not be crushed. Ensure that your pet swallows the capsule whole and does not chew on it. 

Missed a Dose?

Speak with your veterinarian about what to do if you forget to give a dose of tamsulosin. Generally, they may instruct you to give it when you remember, or if it is almost time for your next dose, to skip the missed dose and resume your normal dosing schedule. Do not give extra or double doses.

Tamsulosin Possible Side Effects

Since tamsulosin is not commonly prescribed in pets, the list of known side effects is small. Low blood pressure may be a side effect. Signs of low blood pressure may include:

  • Lethargy

  • Weakness

  • Lack of coordination

  • Collapse

  • Diarrhea

  • Nausea

Human Side Effects

While this is a human prescription medication, there are different dosages and side effects that can occur in humans. If you accidentally ingest a pet medication, call your physician or the national Poison Control Center hotline at 800-222-1222.  


Specific monitoring or routine testing while your pet is on this medication may be recommended by your veterinarian depending on your pet’s individual needs, other medications they may be on and/or the issue that initially caused your pet to be placed on this medication.

Call Your Vet If:

  • Severe side effects are seen (see above)

  • Your pet’s condition worsens or does not improve with treatment

  • You see or suspect an overdose

  • You have additional questions or concerns about the use of tamsulosin

Tamsulosin Overdose Information

Since this medication is not commonly used in pets, there is limited data on overdoses. Symptoms of an overdose may include severely low blood pressure, lethargy, a fast heart rate, vomiting, weakness, lack of coordination, and collapse.

If you suspect an overdose, immediately contact your veterinarian, seek emergency veterinary care, or contact an animal poison control center. Consultation fees often apply.

Pet Poison Helpline (855) 764-7661

ASPCA Animal Poison Control (888) 426-4435

Tamsulosin Storage

Tamsulosin should be stored at controlled room temperatures at or below 77 F, and brief exposure to temperatures from 59 F to 86 F are acceptable. Keep the container tightly closed in order to protect from moisture and light. Always confirm storage requirements by reading the label.

Compounded medications should be stored according to the compounding pharmacy’s label.

Keep out of reach of children and pets.

Tamsulosin FAQs

How quickly does Tamsulosin work in pets?

Tamsulosin starts working in the first few hours after your pet starts the medication, but it may take several days to reach maximum efficacy.

No vet writer or qualified reviewer has received any compensation from the manufacturer of the medication as part of creating this article. All content contained in this article is sourced from public sources or the manufacturer.


Sato S, Ohtake A, Hatanaka T, et al. Relationship between the functional effect of tamsulosin and its concentration in lower urinary tract tissues in dogs. Biological and Pharmaceutical Bulletin. 2007;30(3):481-486

Ohtake A, Sato S, Sasamata M, Miyata K. Effects of tamsulosin on resting urethral pressure and arterial blood pressure in anaesthetized female dogs. Journal of Pharmacy and Pharmacology. 2006;58(3):345-350

Carney HC, Sadek TP, Curtis TM, et al. AAFP and ISFM Guidelines for Diagnosing and Solving House-Soiling Behavior in Cats. Journal of Feline Medicine and Surgery. 2014;16(7):579-598

Kobayashi S, Tomiyama Y, Tatemichi S, Hoyano Y, Kobayashi M, Yamazaki Y. Effects of silodosin and tamsulosin on the urethra and cardiovascular system in young and old dogs with benign prostatic hyperplasia. European Journal of Pharmacology. 2009;613(1-3):135-140

FLOMAX- Tamsulosin Hydrochloride Capsule [US Human Product Label]. Boehringer Ingelheim Pharmaceuticals, Inc.; 2016

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Stephanie Howe, DVM


Stephanie Howe, DVM


Dr. Stephanie Howe graduated from the University of Florida College of Veterinary Medicine in 2011, after receiving a Bachelor of Science...

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