Prazosin is a human blood pressure drug that vets prescribe extra-label for cats, mostly to relax a urethra that spasms after a urinary blockage has been cleared. The goal is to make peeing easier while the urethra settles. What it has not done in studies is lower the chance of the cat blocking again, and one large study found more repeat blockages in cats that got it.
Call your vet now if your cat may be blocked. A cat, especially a male, that keeps going back to the litter box and strains but passes only drops of urine or none, cries while trying, or licks his genital area a lot needs to be seen right away. A blocked cat can die within a day or two, and owners can mistake the straining for constipation. Watch closely in the first weeks after a blockage, when many cats block again.
What Is Prazosin and What Is It Used For in Cats?
Prazosin is an alpha-1 blocker. It blocks the nerve signals that keep certain smooth muscle tight, so that muscle relaxes. In the urinary tract its main target is the internal urethral sphincter: the ring of smooth muscle in the urethra (the tube that carries urine out of the bladder) just past the bladder neck.
Vets reach for it mainly in one situation. A cat with lower urinary tract disease gets a plug of mucus, crystals and cell debris, or a small stone, stuck in his urethra. Male cats are at the highest risk because their urethra is longer and narrower. After the vet unblocks him, usually with a urinary catheter left in for a while, the urethra is often sore and tight, and some cats still struggle to pee even when nothing is blocking the way. The Merck Veterinary Manual notes that this excess sphincter tone is common in cats after an obstruction is relieved, and that an alpha blocker is the drug type used for it.
Less common uses:
- A bladder that won't empty well because the sphincter stays too tight, as with some nerve problems.
- A stone stuck in a ureter, the tube between the kidney and the bladder (see kidney stones in cats). This is a hospital treatment.
- High blood pressure or heart disease. Prazosin widens blood vessels, but Merck notes it is rarely used for heart disease in small animals.
Prazosin treats a symptom, the spasm. It does not dissolve plugs or stones, and it does not treat the inflammation behind them, which is often feline idiopathic cystitis.
There is no veterinary prazosin product, and it is not FDA-approved for animals. Your vet prescribes the human drug extra-label, and a compounding pharmacy usually makes up the small cat doses.
Does Prazosin Prevent Another Blockage? What the Studies Found
For years the hope was that relaxing the urethra would stop cats from blocking again. Three studies have tested it in male cats after a urethral blockage:
Study | Cats | What they got | Re-blocked: prazosin vs no prazosin | Result |
|---|---|---|---|---|
47 male cats, randomized, placebo-controlled | Prazosin or placebo for 1 month | 15% vs 17% during the month; 37% vs 31% at 6 months | No difference | |
65 neutered male cats, randomized, placebo-controlled | Prazosin or placebo for 7 days | 30% vs 18% within 30 days | No significant difference | |
388 cats, observational (vets who always or never used it) | Prazosin for about 14 days, or none | 24% vs 13% within 14 days | More re-blocks with prazosin |
The two placebo trials were small, and their authors said they could have missed a real difference. The Conway study was larger but not randomized, so differences between clinics could explain part of the gap. The 2017 trial also found no difference in the urinary signs owners reported at home. None of the three found that prazosin prevents a repeat blockage, and the Conway authors concluded that vets should not use it routinely for that purpose without a specific reason. A 2023 Veterinary Evidence review agreed and called the evidence weak, with one possible plus: cats on prazosin needed their urinary catheter for a shorter time.
Why it may not help much: the anatomy. Prazosin relaxes smooth muscle only. In a male cat, the far end of the urethra, about 63% to 72% of its length, is striated (skeletal-type) muscle, which prazosin does not relax. That is also where most plugs get stuck, according to a Clinician's Brief review by NC State veterinary researchers. So the drug works on roughly the first third of the urethra, not the part where trouble usually starts.
What this means for you: some vets still prescribe prazosin for a cat that strains after the catheter comes out, and some no longer use it. Both are reasonable readings of thin evidence. If your cat came home with it, give it as directed and ask your vet what they want it to do for your cat. Don't stop it on your own.
Prazosin Dosage Chart for Cats
Prazosin for cats is dosed per cat, not per kg or lb: each study gave the same amount to every adult cat. The first three rows are the amounts the studies used, one row per study.
For information only. These are typical published dose ranges, not a prescription and not medically verified for your pet. Your veterinarian sets the dose after examining your pet. Never start, stop or change a medicine without their OK.
Species | Use | Dose | How often | How long | Route | Source |
|---|---|---|---|---|---|---|
Cat | Urethral spasm after a urinary blockage has been cleared (extra-label) | 0.25 mg per cat | Every 12 hours | 1 month | By mouth | Reineke 2017, placebo-controlled trial |
Cat | Urethral spasm after a urinary blockage has been cleared (extra-label) | 0.5 mg per cat | Every 12 hours | 7 days | By mouth | Hanson 2021, placebo-controlled trial |
Cat | Urethral spasm after a urinary blockage has been cleared (extra-label) | 0.5 to 1 mg per cat | Every 12 hours | About 14 days | By mouth | Conway 2022, what vets generally prescribed (observational study) |
Cat | Stone stuck in a ureter (kidney to bladder), usually in hospital | Set by your vet | Set by your vet | Set by your vet | By mouth | |
Cat | High blood pressure or heart disease (rarely used) | Set by your vet | Set by your vet | Set by your vet | By mouth |
Dose calculator · Prazosin
Prazosin dose calculator for cats
Not a prescription. For information only. These are typical published dose ranges, not a prescription and not medically verified for your pet. Your veterinarian sets the dose after examining your pet. Never start, stop or change a medicine without their OK.
Only give what your vet prescribed.
All doses in this calculator
| Pet | Use | Typical dose | How often | Route | Source |
|---|---|---|---|---|---|
| Cat | Urethral spasm after a urinary blockage has been cleared (extra-label) | 2 to 10 kg (4.41 to 22 lb): 0.25 to 1 mg per catStudies used 0.25 to 1 mg per cat; your vet picks the amount. Reineke 2017: 0.25 mg for 1 month. Hanson 2021: 0.5 mg for 7 days. Conway 2022: 0.5 to 1 mg for about 14 days, what vets generally prescribed. Extra-label: your vet decides. The amount is per cat, not per kg or lb, so it is the same at any weight; 2 to 10 kg (4.4 to 22 lb) is only the calculator's display range. Give with food. Human capsules start at 1 mg, so smaller doses are compounded by a pharmacy: don't open or split a capsule at home. None of the studies found fewer repeat blockages with prazosin. A male cat straining with little or no urine is an emergency: call now. | Every 12 hours | By mouth | Reineke 2017; Hanson 2021; Conway 2022 |
Only give what your vet prescribed, at the dose they set.
Call now if your cat got more than prescribed, or collapses, faints, is very weak or wobbly, has a racing heart or seems feverish or frantic after a dose: call your vet, the ASPCA Animal Poison Control Center at (888) 426-4435, or the Pet Poison Helpline at (855) 764-7661.
Two things the chart can't show. First, human prazosin capsules come in 1 mg, 2 mg and 5 mg, so most cat doses are below the smallest capsule and a pharmacy compounds them into smaller capsules or a liquid. Don't open or split a human capsule at home. Second, the two "set by your vet" rows have no numbers on purpose: Merck's heart disease range covers dogs and cats together and is rarely used, and its ureteral stone amount is per kg and given in hospital, so neither is a home dose.
How to Give Prazosin to Your Cat
- Give it with food, at about the same times each day.
- Use the form your cat takes best. If pills are a fight, ask your vet or pharmacist about a flavored liquid. Stress at medicine time is worth solving, because stress plays a part in urinary flare-ups.
- Finish the course your vet set, and ask before stopping early.
- Wash your hands after handling any medicine.
How long does it take to work? Prazosin starts acting within 1 to 2 hours, and the effect wears off within about 24 hours. It can last longer in cats with kidney or liver disease. You won't see the drug working directly. What you are watching for is a cat that pees comfortably and in normal amounts.
Side Effects of Prazosin in Cats
Many cats show no obvious side effects, and the most common one is tiredness. Cats in the trials also had drooling, diarrhea, poor appetite and smelly stool; drug references add weakness, dizziness, vomiting, constipation and a raised third eyelid (the pale membrane in the inner corner of the eye). Because prazosin lowers blood pressure, the effects to take seriously are signs of it dropping too far.
What you see | What to do |
|---|---|
A little sleepier than usual, mild drooling, softer stool, eating a bit less | Mention at your next visit (call sooner if it is severe, getting worse or not going away) |
Vomiting, diarrhea that keeps going, not eating, very sleepy | See your vet soon (call the same day) |
Collapse or fainting, severe weakness or wobbling, a racing heart, fever, hyperactivity or agitation, an overdose | Call now |
Straining in the litter box with little or no urine, crying while trying to pee | Call now: this is a blockage until proven otherwise, not a side effect |
If your cat seems weak or wobbly after a dose, call your vet.
Who Should Not Take Prazosin, and Drug Interactions
Tell your vet if your cat:
- has had a reaction to prazosin before;
- has kidney disease or low blood pressure (prazosin is used with caution in both);
- is pregnant or nursing, so you can talk through the risks.
Several other drugs also lower blood pressure. The human label warns of an added drop when prazosin is combined with beta-blockers, sildenafil-type drugs (PDE-5 inhibitors) and other blood pressure medicines. Give your vet a full list of everything your cat takes, including supplements.
Missed Dose and Storage
Missed a dose? Give it when you remember. If it is almost time for the next one, skip the missed dose and carry on as normal. Never give two doses at once.
Storage: keep capsules in the original bottle at room temperature, away from moisture and light. Compounded liquids and capsules can have their own storage instructions and expiry date, so follow the pharmacy label. Give prazosin only to the cat it was prescribed for, and keep all medicines where children and pets can't get at them.
Helping Your Cat Recover and Avoid Another Blockage
Prazosin is a small part of the plan. Treating the underlying problem and making peeing easy and stress-free do more to prevent another blockage. The Cornell Feline Health Center recommends:
- Fresh, clean water available at all times.
- One more litter box than the number of cats, kept clean, in quiet, safe spots.
- Steady routines and less stress, which plays a big part in feline idiopathic cystitis.
- Small, frequent meals of the diet your vet recommends.
Keep watching the litter box closely for the first weeks home. More trips, smaller or fewer clumps, or straining with little or no urine: call your vet right away.
Questions to ask your vet:
- What do you want prazosin to do for my cat, and for how many days?
- What should I see in the litter box if he is doing well, and what means come straight back?
- Does he need a special diet, and for how long?
- Should anything change with his other medicines?
Prazosin for Cats: Quick Questions
Does prazosin make cats sleepy? It can; lethargy is the side effect reported most often. A cat that is hard to rouse, weak or wobbly needs a call to your vet.
Will prazosin stop my cat from blocking again? Studies so far say probably not. Preventing another blockage depends on the underlying cause, water intake, diet and stress, and on catching straining early.
Can my cat take my prazosin? No. Human capsules start at 1 mg, more than many cat doses, and only your vet can say whether your cat should take it at all.
Final Thoughts
Prazosin can ease a urethra that spasms after a blockage, but the best studies so far say it does not prevent the next one. Give it exactly as prescribed, with food, and treat any return of straining with little or no urine as an emergency. Our guides to diarrhea in cats and constipation in cats cover gut problems that can sit alongside urinary trouble.
Originally written by Dr. Kaylee Ferreira. Updated by the I Love Veterinary editorial team.
Sources
- Reineke EL et al. The effect of prazosin on outcome in feline urethral obstruction. J Vet Emerg Crit Care, 2017
- Hanson KR et al. Effect of prazosin on feline recurrent urethral obstruction. J Feline Med Surg, 2021
- Conway DS et al. Prazosin administration increases the rate of recurrent urethral obstruction in cats: 388 cases. JAVMA, 2022
- Veterinary Evidence: The efficacy of prazosin in reducing the risk of recurrent urethral obstruction in male cats (Hearne, 2023)
- Clinician's Brief: Effect of Prazosin on Recurrent Urethral Obstruction in Cats
- Merck Veterinary Manual: Pharmacotherapeutics in Urine Retention in Animals
- Merck Veterinary Manual: Urethral Obstruction in Small Animals
- Merck Veterinary Manual: Obstructive Uropathy in Dogs and Cats
- Merck Veterinary Manual: Vasoactive Drugs for Use in Animals
- Cornell Feline Health Center: Feline Lower Urinary Tract Disease
- VetMedux client information sheet: Prazosin (2024)
- PetMD: Prazosin
- DailyMed: Prazosin hydrochloride capsules (human label)
This article is for general information and is not a substitute for advice from your veterinarian.
