Probiotics for cats are a narrow, real tool: 10^8 to 10^9 CFU a day for 3 to 7 days during an episode of acute diarrhea, given 2 hours away from any antibiotic, and only with a strain that has actual data behind it. They do not treat panleukopenia, they do not fix a food allergy, and they do not replace a vet visit that is already 48 hours overdue.
What the evidence actually supports
Most products sold for cats have never been tested in cats. The published cat data covers a handful of strains, mostly for acute diarrhea, stress colitis in shelter cats, and recovery after a course of antibiotics. The effect size in those settings is modest: stool firms up 1 to 2 days sooner in some cats, and roughly 1 in 3 shows no measurable change at all.
What probiotics do not do is just as consistent. There is no good evidence that they shorten viral enteritis, resolve chronic enteropathy, or replace metronidazole or a hydrolyzed diet. A cat losing weight over 3 to 4 weeks with soft stool needs bloodwork, a B12 check and possibly an ultrasound, not a 30 USD tub of powder.
EF versus GG: the two strains you will actually find
Enterococcus faecium SF68 is the strain with cat-specific work behind it, including shelter and stress-diarrhea trials, and it is the one most veterinary brands use. It survives gastric acid well, colonizes transiently for 5 to 10 days, and is stable at room temperature, which matters if the tub sits in a kitchen for 8 weeks.
Lactobacillus rhamnosus GG is the strain with the deepest evidence base overall, but almost all of it is pediatric human data on rotavirus diarrhea at 10^9 to 10^10 CFU a day. In cats it is used by extrapolation. It is also less stable: many GG products need refrigeration, and CFU counts in opened tubs fall 1 to 2 logs within 4 to 6 weeks at 25 C.
Strain comparison at a glance
| Strain | CFU per day | Cat data | Fridge needed | 7-day cost (USD) |
|---|---|---|---|---|
| E. faecium SF68 | 10^8 | Yes, acute and stress diarrhea | No | 8–14 |
| L. rhamnosus GG | 10^9–10^10 | Extrapolated from human trials | Usually | 6–12 |
| B. animalis subsp. lactis | 10^8–10^9 | Mostly dog data | Sometimes | 7–15 |
| Multi-strain, 8-12 strains | 10^6–10^7 | None per strain | Varies | 10–20 |
| S. boulardii yeast | 10^9–10^10 | Limited, mostly human | No | 12–22 |
The last two rows are why the label matters: 12 strains at 10^6 CFU each is 10^7 total, which is 10 to 100 times below the working range, and a yeast is not a bacterium, so it survives antibiotics and can be given at the same time.
Dose: 10^8 to 10^9 CFU a day
An adult cat gets 10^8 to 10^9 CFU a day, a kitten under 6 months about 10^8, and a cat over 6 kg the top of the range. Split it into 2 doses only if the total is above 5 x 10^8; below that, 1 dose is easier to keep down.
Read the number on the label twice. A count stated as CFU per gram at manufacture is not CFU at expiry, and a 5 x 10^9 CFU/g powder that loses 1 log in 6 months and is dosed at 0.5 g delivers 2.5 x 10^8, still inside the range. A 10^8 CFU/g product dosed at 0.5 g delivers 5 x 10^7, which is below it.
Three to seven days, then reassess
For acute diarrhea the usual course is 3 to 7 days. Most cats that respond show firmer stool by day 3, with the stool score moving from 5 or 6 on a 7-point scale to 3 or 4. If there is no change by day 7, stop; continuing for 3 more weeks adds cost without adding effect. After a 5 to 7 day antibiotic course, a 2 to 4 week course is the common practice, started the same day the antibiotic ends.
Stop and call the vet rather than extend if diarrhea lasts beyond 48 hours in a kitten, beyond 72 hours in an adult, or if any blood appears.
The 2-hour gap from antibiotics
Give the antibiotic first, then the probiotic 2 hours later. Both in the same mouthful means the antibiotic meets a concentrated dose of the live culture in a small volume, and the kill rate is high. The same 2-hour rule applies to probiotic and antibiotic in food: do not stir both into one 30 g portion.
Two more handling rules decide whether the dose arrives alive. Do not mix into food above 40 C, which kills most Lactobacillus within 1 to 2 minutes, and do not leave the mixed portion out for 2 hours, since the CFU count in a wet meal at 25 C drops measurably within 60 to 90 minutes. Mix into a 25 to 30 C meal and serve immediately.
When a probiotic is the wrong tool
Skip it, or use it only alongside treatment, in these cases: a kitten under 8 weeks, a cat on chemotherapy or steroids, a cat with a central line, suspected panleukopenia, a foreign body, or diarrhea with vomiting more than 3 times in 12 hours. Immunocompromised cats have developed bloodstream infections from probiotic strains, which is why veterinary products carry that warning.
Parasites are the other trap. Giardia and Tritrichomonas cause soft stool that no probiotic fixes; a 3-day fecal PCR at 60 to 120 USD settles it faster than 2 weeks of guessing. Probiotics are also not a substitute for the 7 to 10 day transition when changing food, though they are often used alongside it.
Cost per course
Veterinary sachets run 0.80 to 1.50 USD each, so a 7-day course is 6 to 11 USD at 1 sachet a day. A 30-capsule tub is 25 to 45 USD and covers 30 to 60 days. Human GG sachets at 0.50 to 1.00 USD are the cheap route and are the same organism, though the CFU per sachet is often 10^9 to 10^10, above the cat range, so halve it.
Against that, a single vet visit for diarrhea is 80 to 200 USD before fecal testing, so a 10 USD course that shortens an episode by 1 to 2 days is worth trying once, not repeatedly.
What to do this week
- Pick one strain, not a 12-strain blend, at 10^8 to 10^9 CFU per dose.
- Check the CFU number is at expiry, not at manufacture.
- Dose for 3 to 7 days, then decide: continue only if stool score improved.
- Space 2 hours from any antibiotic, and never mix into food above 40 C.
- Book the vet instead of a second tub if diarrhea passes 48 hours in a kitten or 72 in an adult.
If the diarrhea started within 2 weeks of a food change, read the 7 to 10 day transition plan before adding anything else; if you are switching to a higher-protein, lower-carb formula, the numbers are in carb content in cat food. Ongoing food and supplement spend goes into the cost calculator.
Frequently asked
How long should a cat stay on probiotics?
3 to 7 days for an acute episode, then reassess. Most cats that respond show firmer stool by day 3, with the stool score dropping 1 to 2 points on a 7-point scale. After a 5 to 7 day antibiotic course, 2 to 4 weeks is common practice, starting the day the antibiotic stops.
Can I give probiotics at the same time as antibiotics?
No. Give the antibiotic first and the probiotic 2 hours later, and do not mix both into one 30 g portion of food. The yeast Saccharomyces boulardii is the exception, since it is not a bacterium and survives antibiotics, so it can go at the same time.
Do probiotics help with diarrhea in cats?
Modestly, and only for some causes. Cat data supports a 1 to 2 day shorter course in acute and stress-related diarrhea, with about 1 in 3 cats showing no measurable change. They do not treat viral enteritis, giardia, a foreign body or chronic enteropathy, and a cat losing weight needs tests, not supplements.