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Prazosin is a human blood pressure drug that relaxes smooth muscle in the urethra. In cats, it's given after a urinary blockage has been cleared, at 0.25 to 0.5 mg per cat every 12 to 24 hours, to try to stop the cat from blocking again.
Most cats we unblock still go home on it. The evidence that it prevents a second blockage is weak, though. Two placebo-controlled trials found no benefit, and a study of 388 cats found more re-blocking in the cats that received it.1,2,3
A cat straining in the litter box and passing little or no urine needs a vet right away. It happens mostly in male cats, and a complete blockage can kill a cat in under 24 to 48 hours.4 Prazosin can't open a blocked urethra, so don't give leftover doses and wait to see if it helps. Call us at (519) 896-0532, or see our emergency care page for after-hours help.
Key Takeaways
It's started once a urethral blockage has been cleared, almost always in a male cat. The aim is to relax the urethra so it doesn't block again, which happens in 15% to 40% of cats.7
Prazosin is an alpha-1 blocker. It stops the nerve signal that tightens smooth muscle, which relaxes the urethra. It also widens blood vessels, which is how it lowers blood pressure in people.5
Dogs take it too, mainly for reflex dyssynergia, where the urethra tightens while the bladder is trying to empty.8
Interestingly, no study has shown that it does, and one large study found the opposite.
| Study | Design | Re-blocked on prazosin | Re-blocked without it |
|---|---|---|---|
| Hetrick and Davidow, 20139 | Records review of 192 cats, compared with phenoxybenzamine | 18% within 30 days | 39% on phenoxybenzamine |
| Reineke et al., 20171 | Randomized, placebo-controlled, 47 cats | 15% during a 1-month course | 17% on placebo |
| Hanson et al., 20212 | Randomized, placebo-controlled, 65 cats | 30% within 30 days | 18% on placebo |
| Conway et al., 20223 | Observational, 388 cats | 24% within 14 days | 13% with no prazosin |
The only favourable result compared prazosin with phenoxybenzamine, an older and less selective drug. That study had no untreated group, so it can't say whether either drug beats no treatment.2,9
Neither placebo trial found a benefit. Both were too small to rule one out, and their authors said so. In the second, more cats blocked again on prazosin than on placebo, though a gap that size could have been chance.1,2
Only the Conway study found harm. It compared cats seen by vets who always prescribe prazosin with cats seen by vets who never do. That design isn't randomized, and differences between those practices may explain part of the gap. The authors still concluded that prazosin shouldn't be used to prevent re-blocking without a specific reason.3
A 2023 evidence summary of the two trials and the Conway study reached the same place. Prazosin doesn't reduce the risk of re-blocking in the first 30 days, and the evidence either way is weak.10
Prazosin relaxes smooth muscle, and in a male cat, smooth muscle lines only about the first third of the urethra. The rest is skeletal muscle, which prazosin doesn't relax.2,6
Blockages usually lodge at the far end, inside the penis, where the urethra is narrowest.11 That stretch is skeletal muscle.
Urethral spasm, which prazosin is given to ease, may not be a big factor either. Researchers who measured urethral pressure in six recently blocked cats concluded that spasm played only a minor role.12 Six cats is a tiny sample, but the finding lines up with the trial results.
The dose is 0.25 to 0.5 mg per cat, by mouth, every 12 to 24 hours. It's a per-cat dose, so it isn't worked out from your cat's weight.
| Detail | Prazosin for cats |
|---|---|
| Dose | 0.25 to 0.5 mg per cat |
| How often | Every 12 to 24 hours |
| How long | A short, set course. Published courses have run from 5 days to 1 month.1,2,3,6 |
| Form | Compounded to your cat's dose, since the smallest human capsule is 1 mg5 |
Give every dose until the course is finished, even once your cat seems back to normal. If you miss one, give it when you remember unless the next dose is nearly due. Never give two at once.
Every unblocked cat should also go home on pain relief for 5 to 7 days, most often buprenorphine.6
Owners in the first placebo trial reported lethargy, drooling, diarrhea, poor appetite and foul-smelling stool.1 In the second, no cat developed low blood pressure and owners reported no specific side effects.2
Diarrhea or a poor appetite deserves a phone call in a cat that's just been unblocked. A cat that isn't eating or drinking well gets dehydrated, and drinking plenty is one of the few things shown to lower the risk of another blockage.6
Low blood pressure is the main risk, because prazosin widens blood vessels. In people, fainting is most likely within 90 minutes of the first dose.5 In a cat, watch for weakness, wobbliness or collapse, especially over the first few doses, and call us if you see it.
Cats with low blood pressure, and cats that have reacted to prazosin or a related drug from the same chemical family, the quinazolines.5
Any other drug that lowers blood pressure adds to its effect, and together they can drop it too far.5 In cats, that means amlodipine, telmisartan and benazepril, as well as diuretics such as furosemide, beta-blockers such as atenolol, and sildenafil.5
Tell us about every medication your cat takes, including treatment for heart disease such as hypertrophic cardiomyopathy. The 2021 placebo trial excluded cats on heart or blood pressure medication, so there's no trial data on how those cats do.2
Get more water into your cat. Of the factors studied in cats that block again, drinking more water was the only one independently linked to a lower risk.6
Wet food is the simplest way to do that. More water bowls, more litter boxes and less stress at home also help.6,7
Watch closely for the first two weeks. In the studies above, much of the re-blocking happened in that window, some of it before the cat had even gone home.2,3
A second blockage looks like the first: straining, repeated trips to the litter box, little or no urine, and crying out as it gets worse.4 Don't wait to see if it passes. Call us right away.
Call us right away. If we're closed, go to the Emergency Veterinary Clinic of Waterloo Region in Cambridge, which is open 24 hours. Straining while on prazosin can mean the urethra is blocking again, and more prazosin won't open it, so don't give an extra dose while you wait. A complete blockage can become fatal in under 24 to 48 hours.
Don't stop it on your own. The studies describe groups of cats, and whether it's right for yours is something to talk through with us. Call first, whether your cat is having side effects or you'd simply rather not continue.
Yes. Prazosin is a human blood pressure medication, and there's no version made for animals. The smallest human capsule is 1 mg, which is two to four times a cat's dose, so a pharmacy compounds it to the right strength. Don't split a human capsule at home or give your cat a family member's prescription.
Long-term use hasn't been studied in cats. The published courses ran from 5 days to a month. For a cat that keeps blocking, the options with evidence behind them are more water, wet food and, for some male cats, a surgery called a perineal urethrostomy that widens the urethra.
Yes. In dogs it's used mainly for reflex dyssynergia, where the urethra tightens while the bladder is trying to empty, so the dog starts to urinate and then stops. The dose for a dog is different from a cat's and is set by your vet. Never give one pet's prazosin to another.
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