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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Comfortis (spinosad) is a prescription oral flea treatment that the US label approves for cats and kittens at least 14 weeks old and 4.1 pounds. It is given once a month with food, and in the United States it may only be used by or on the order of a licensed veterinarian. The label reports that vomiting was the most common reaction in a controlled field study, and FDA’s post-approval list includes neurologic events such as seizures. The sections below explain what the US label says about dosing, safety, and who should be involved in the decision, and they note where the evidence is limited.
The information here comes from the US FDA feline labeling-change summary dated June 27, 2024 and the Comfortis US label on DailyMed (labeler Elanco US Inc., updated November 6, 2024). Labels change, so compare any figure against the current label or your veterinarian’s copy before acting on it.
Who the US label covers
In the United States, Comfortis is labeled for the prevention and treatment of Ctenocephalides felis (cat flea) infestations in cats and kittens that are at least 14 weeks old and weigh at least 4.1 pounds. The label directs a monthly schedule. Cats outside those age and weight limits are outside the labeled use, and any decision to treat them is a matter for the veterinarian.
The label is a US document. The European Medicines Agency’s public assessment report states that the EU marketing authorisation was withdrawn at the authorisation holder’s request, so a product page or pharmacy listing from another country should not be read as confirmation of local availability.
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Dosage and tablet strengths
The US label sets the minimum dose at 22.5 mg per pound (50 mg per kg) of body weight, given orally once a month. For cats, the label groups weights into three tablet strengths:
| Cat weight (US label) | Spinosad per tablet | Labeled tablets |
|---|---|---|
| 4.1 to 6 lb | 140 mg | One tablet |
| 6.1 to 12 lb | 270 mg | One tablet |
| 12.1 to 24 lb | 560 mg | One tablet |
| Over 24 lb | Not a single cat tablet; the label directs use of an appropriate combination | As directed by the label and veterinarian |
These bands are a reference for reading the label, not a substitute for weighing the cat on the day of dosing. A cat near a boundary, or one that has gained or lost weight since the last dose, is a case to discuss with your veterinarian rather than to resolve from the table alone.
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How to give a dose
The label describes these practical points:
- Timing with food. Give the chewable tablet with food for the best effect. The label allows it to be offered just before or after a meal, placed in food, or given like another tablet.
- If the cat vomits within one hour. The label says to redose with another full dose.
- If a dose is missed. The label says to give the dose with food and then resume the monthly schedule.
The label’s vomiting and missed-dose instructions are written for typical cases. If your cat is ill, is on other medications, or the vomiting is repeated, call your veterinarian before giving another tablet instead of working it out at home.
Side effects in the controlled field study
The US label describes a well-controlled field study of 211 cats: 139 received Comfortis and 72 received an active topical control, each given once monthly for three treatments. The label reports no serious adverse reactions attributed to Comfortis in that study. Vomiting was the most frequently reported reaction, and the month-by-month rates were:
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| Study month | Vomiting, Comfortis-treated cats | Vomiting, active topical control |
|---|---|---|
| Month 1 | 14.4% | 1.4% |
| Month 2 | 14.8% | 1.4% |
| Month 3 | 13.6% | 4.5% |
Other reactions reported in more than 1% of cats in at least one month included lethargy, decreased appetite (anorexia), weight loss, and diarrhea. These numbers describe one 90-day study of cats in that setting. They are not a general incidence rate and cannot predict what will happen to a particular cat.
Events reported after approval
After a product is on the market, veterinarians and owners report side effects to FDA, and the report list is more varied than the trial list. FDA’s June 27, 2024 feline summary lists the following reported events, in decreasing order of reporting frequency: vomiting, depression or lethargy, anorexia, behavioral changes, pruritus (itching), diarrhea, ataxia (loss of coordination), hyperactivity, hypersalivation, panting, vocalization, dyspnea (difficulty breathing), twitching, and seizures.
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Two limits apply to this list. Not every event is reported, so the list does not measure how often something happens. The reports also cannot establish that Comfortis caused an event. FDA notes that some reports involved death, including cats with heart disease, some of which had not been diagnosed before treatment. A report of a death in a cat that received Comfortis does not show that the drug was the cause.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Ivermectin and heartworm preventives
The label handles two different situations differently, and they should not be combined.
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- Extra-label ivermectin. The label advises caution when Comfortis is given with ivermectin that is not used as labeled. Post-approval reports in cats given this combination included ataxia, trembling or twitching, seizures, dilated pupils (mydriasis), disorientation, and transient blindness.
- Heartworm preventives. The label states that post-approval experience continues to support safety when Comfortis is used with heartworm preventatives given according to their label directions. That statement covers labeled use. It is not reassurance about extra-label ivermectin.
If your cat is already receiving ivermectin for any reason, tell your veterinarian before starting Comfortis so the two can be reviewed together.
Breeding, pregnant, and nursing cats
The label lists no known contraindications. It also states that the safe use of Comfortis in breeding, pregnant, or lactating cats has not been evaluated. That is a gap in the evidence, not a finding of safety or harm. For a cat that is pregnant, nursing, or intended for breeding, the decision should be made by the veterinarian with the specific cat in front of them.
Prescription status and where it is authorized
In the United States, federal law restricts Comfortis to use by or on the order of a licensed veterinarian. Buying the product without a veterinary prescription is outside the labeled use. Availability outside the US differs, and the EU authorisation has been withdrawn, so check your local regulator and your veterinarian before assuming a product can be obtained.
When to call your veterinarian
- Vomiting within an hour of a dose, or repeated vomiting on later days
- Lethargy, reduced appetite, weight loss, or diarrhea that does not settle
- Tremors, twitching, unsteady walking, seizures, or sudden changes in behavior
- Difficulty breathing, panting that is unusual for the cat, or excess drooling
- Any use alongside ivermectin, or any question about a pregnant, nursing, or breeding cat
- Uncertainty about the correct tablet strength after a weight change
Call promptly if signs are severe or come on quickly. Bring the product packaging and the date and time of the last dose to the visit.
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