Eighty-three cats with hypertrophic cardiomyopathy (HCM) and recently controlled congestive heart failure were randomly assigned to pimobendan or a placebo for 180 days. At the end, 30.8% of the treated cats and 35.1% of the placebo cats had met the study's definition of success — a 4.3-point gap in the wrong direction, with P = .75. That result is the most useful thing an owner can carry into a conversation about a drug no regulator has licensed for cats and that costs roughly $50–65 a month.
What the 180-day trial actually measured
Published in the Journal of Veterinary Internal Medicine in 2021, the study enrolled cats that had already had at least one episode of congestive heart failure and had been stabilised on furosemide. Forty-three cats received pimobendan at 0.30 mg/kg every 12 hours, 39 received placebo, and one received no medication; all continued furosemide at under 10 mg/kg per day, with or without clopidogrel. Thirty of the cats had a dynamic left ventricular outflow tract obstruction (LVOTO); 53 did not.
Success did not mean survival. It meant the cat was still in the study on day 180 without the furosemide dose having been raised — a proxy for "the heart failure stayed controlled". Only 53.9% of cats (41 of 76) reached day 180 at all.
The results, in one table
| Outcome | Pimobendan | Placebo | Statistical note |
|---|---|---|---|
| Primary success at 180 days | 30.8% (12/39) | 35.1% (13/37) | P = .75; OR 0.85 |
| Non-obstructive subgroup | 32.0% (8/25) | 18.2% (4/22) | OR 2.12, CI 0.54–8.34 |
| LVOTO subgroup | 28.6% (4/14) | 60% (9/15) | OR 0.27, CI 0.06–1.26 |
| Cardiac death or euthanasia | 10% (4 cats) | 16% (6 cats) | Not significant |
| Furosemide dose escalation | 13% (5 cats) | 11% (4 cats) | Not significant |
| Gradient up more than 25 mmHg after first dose | 9.3% (4/43) | — | Safety signal in LVOTO |
Why the obstructive subgroup behaved differently
Pimobendan is an inodilator: it increases contractility while dilating systemic vessels. In a cat whose outflow tract is already narrowed, a harder squeeze plus lower pressure downstream can worsen the obstruction. Four of the 43 treated cats (9.3%) crossed the pre-set threshold of a 25 mmHg rise in systolic gradient after the first dose, and three of them were removed early — a protocol decision the authors later flagged as a possible bias against the drug in that subgroup.
The non-obstructive subgroup pointed the other way: 32% versus 18.2%. The confidence interval, 0.54 to 8.34, still crosses 1, so this is a hypothesis rather than a finding. In a disease where nothing else has passed a randomised trial, it is at least a hypothesis worth testing.
Where the "pimobendan works" story came from
Two earlier papers are usually cited. A 2014 retrospective of 27 cats with HCM and heart failure reported a median survival of 626 days with pimobendan against 103 days without. A 170-cat open-label series found the drug well tolerated, with adverse effects in about 3%. Neither was randomised. In the 27-cat study the groups were not contemporaneous, clopidogrel use was 63% in the pimobendan group versus 22% in controls, and atenolol use ran the other way at 33% versus 11%. Both drugs change outcomes by themselves. That is confounding, not evidence.
Why dog data does not transfer
Pimobendan earned its reputation in dogs with myxomatous mitral valve disease. The EPIC trial, 360 dogs with preclinical disease and an enlarged heart, cut the risk of heart failure or cardiac death by 36% and stretched the symptom-free period by about 15 months. Those dogs had a volume-overloaded, poorly contracting ventricle. A cat with HCM has a stiff, thick ventricle that fills badly while squeezing normally or better than normal. HCM is also common: roughly 1 in 7 cats is affected, and a screening study of 780 apparently healthy cats put prevalence near 15%.
When vets still prescribe it
Two situations survive the trial. The first is heart failure caused by a cardiomyopathy with genuine systolic dysfunction — dilated, restrictive, arrhythmogenic right ventricular, or an unclassified phenotype — where the inotropic effect is mechanistically defensible. The second is a time-limited trial in a cat with refractory congestion and no meaningful LVOTO, with the owner told plainly that the 180-day data showed no average benefit and that the goal is easier breathing, not a longer life.
The drug should not be given where there is a fixed obstruction — aortic or pulmonic stenosis — because squeezing harder cannot open a rigid valve.
Dose, cost and what to monitor
Clinical practice has settled on 1.25 mg every 12 hours, about 0.25 mg/kg for a 4–5 kg cat; the trial used 0.30 mg/kg. No product is licensed for cats, though an oral solution approved for dogs (10 mg/ml) is now sold in the UK, EU, US, Japan and Australia and is easier to measure into a small patient.
- Cost: 1.25 mg chewable tablets run about $0.83–1.05 each ($0.83–0.99 at discount pharmacies, $52.49 for 50 at Chewy), so 60 tablets a month for a 4–5 kg cat costs roughly $50–65.
- Breathing: count resting breaths for 60 seconds. Under 30 per minute is the target; a sustained rise over two days is a phone call, not a wait.
- Circulation: gum colour, blood pressure and heart rate at each recheck; weakness or collapse after a dose is a stop-and-call event.
- Murmur: a new or louder murmur suggests a rising gradient — ask for an echo rather than a dose change over the phone.
- Clots: HCM cats also throw clots; sudden hind-limb paralysis is a separate emergency with its own clock.
What to ask before you fill the prescription
Ask whether the echo showed an outflow gradient, and if so how large. Ask what the drug is meant to achieve in this particular cat and how you will know within 30 days whether it is working. Ask what would make the clinic stop it. Then ask for the written monthly cost including rechecks, because the tablet is the cheap part of the decision.
This article summarises published veterinary research and does not replace a diagnosis or prescription from your own veterinarian. Dose, monitoring interval and stopping rules must be set by the clinician who has examined the cat.
Terms mentioned in this article
Frequently asked
Does pimobendan help cats with hypertrophic cardiomyopathy?
In the only randomised trial, 83 cats with HCM and recently controlled heart failure showed no overall benefit at 180 days: 30.8% of treated cats met the success endpoint versus 35.1% on placebo (P = .75). Cats without outflow obstruction did better on the drug (32% versus 18.2%), but that difference was not statistically significant. It remains a reasonable option for systolic-dysfunction phenotypes and for selected cats without significant LVOTO, decided case by case.
Why is pimobendan dangerous in cats with outflow tract obstruction?
Pimobendan increases contractility and lowers systemic vascular resistance at the same time, so a narrowed outflow tract can become more obstructed. In the 2021 trial, 4 of 43 treated cats (9.3%) had a systolic gradient rise of more than 25 mmHg after the first dose, and three were withdrawn early. A new or louder murmur, weakness or collapse after starting the drug warrants an echo before the next dose.
How much does pimobendan cost for a cat each month?
A 4–5 kg cat takes 1.25 mg every 12 hours, roughly 60 tablets a month. At $0.83–1.05 per 1.25 mg tablet ($52.49 for 50 at Chewy), the drug alone runs about $50–65 a month, before rechecks, blood pressure checks and echocardiograms. No pimobendan product is licensed for cats, so the prescription is off-label and the monitoring plan should be agreed in writing.