Flea control for a cat over 10 comes down to 4 rules: treat every 30 days, never use a product below the 1.5 kg body weight floor, never put a pyrethroid on a cat, and treat every cat in the house within 48 hours. The drug choice and the dose are a prescription decision, set by body weight and by whatever the kidneys and liver can clear, which is why the label on the box is a starting point and not the plan. The 3 things that go wrong in older cats are underdosing a 2.8 kg cat with a product made for 4 kg, using a dog pipette on a cat, and treating one cat while the other 2 keep the house infested.
Why an old cat is not a small adult cat
Three changes matter. Skin gets thinner and loses elasticity, so a spot-on spreads over a larger area and is absorbed faster. Body fat drops, and fat-soluble drugs that used to sit in fat now reach a higher peak concentration. Kidney and liver clearance falls, which lengthens how long a drug stays in the body at a level that can cause signs. A cat that has lost 800 g since last year may now sit in the dose band below the one printed on the box it has used for 5 years, and continuing the same pipette is an overdose in everything but name.
The 1.5 kg floor
Most spot-on products are licensed from 1.5 kg or 2 kg upward, and a few from 1 kg with a reduced volume. Under 1.5 kg there is usually no licensed spot-on dose, and the options are a prescription oral product, a prescription spray dosed by body weight, or mechanical control plus treating the environment. A cat at 1.4 kg after a month of poor appetite is not a candidate for a split pipette; splitting an ampoule gives an unknown volume, uneven absorption and no safety data. Weigh the cat monthly to 0.1 kg, write it on a card, and take the number to the appointment.
Why pyrethroids are banned in cats
Permethrin, phenothrin, deltamethrin, cypermethrin and etofenprox are the pyrethroid and pyrethrin-type insecticides found in most dog spot-ons, some dog shampoos and many household sprays. Cats clear these compounds poorly because they lack the glucuronidation pathway dogs use, so a dose that is routine for a 10 kg dog can be a seizure dose for a 4 kg cat. Signs usually start 2 to 12 hours after contact: twitching ears and facial muscles, drooling, tremors that progress to generalized seizures, fever above 40 C, and in severe cases breathing difficulty. There is no antidote. Treatment is decontamination, seizure control, fluids and 24 to 72 hours of monitoring, and the bill for that is 5 to 20 times the cost of the correct product. Read the active ingredient on every box, including the ones bought for the dog, and never apply a dog product to a cat to save 20 dollars.
Monthly means every 30 days
Flea products are labeled for a 30-day interval, and the protection curve drops before the next dose is due. Stretching to 6 weeks leaves a 12-day gap in a house with an active infestation, and the flea life cycle runs 21 to 28 days indoors at 20 to 25 C, which is enough for a new generation to mature in that gap. Put the date in a calendar, not in memory, and treat on the same day each month. If the cat is bathed, wait 48 hours before and after application, and use a shampoo that does not strip the product.
| Body weight (kg) | Typical band | Interval (days) | Re-weigh (days) |
|---|---|---|---|
| 1.0 to 1.4 | Prescription only, no standard spot-on | 30 | 14 |
| 1.5 to 2.5 | Smallest labeled pipette | 30 | 30 |
| 2.6 to 4.0 | Second band, most common for seniors | 30 | 30 |
| 4.1 to 6.0 | Third band | 30 | 30 |
| Over 6.0 | Largest band, or 2 pipettes by prescription | 30 | 30 |
All cats within 48 hours
Fleas do not stay on the cat that carries them. Eggs fall off within 8 hours of feeding, larvae live in carpet and bedding, and adults emerge and jump onto whichever host is closest. In a 3-cat house, treating 1 cat on Monday and the others on Friday leaves 4 days in which the untreated 2 restock the environment, and the treated cat is reinfested within 2 to 3 weeks. Treat every cat, including the indoor-only one that never goes out, inside a 48-hour window, and treat the dog on the same day. If a cat cannot be treated for a medical reason, say so out loud and plan for it: the household will need environmental control and monthly combing, not a different pipette.
The house is half the job
Vacuum floors, rugs and the cat beds on the same day the cats are treated, then seal the bag and bin it outside. Wash bedding at 60 C or above. A flea pupa in carpet can wait months for a host, so a single deep clean does not finish the job; vacuum every 2 to 3 days for 3 weeks, which is long enough to catch 2 generations. Indoor-only cats in a flat on the 8th floor still get fleas, usually carried in on shoes or on a visiting dog, so indoor status is not a reason to skip the monthly dose.
When to call the clinic
Call the same day for tremors, drooling, vomiting within 2 hours of application, a cat that cannot stand, or a temperature above 40 C. Call within 24 hours for itching, redness or hair loss at the application site, or if the cat has been exposed to a dog product. Call before the next dose if the cat's weight has changed by more than 10 percent, if a new medicine was started, or if fleas are still visible 14 days after a correctly applied dose, because that suggests resistance or a missed source rather than a product that needs doubling.
Five steps this month
- Weigh the cat to 0.1 kg and confirm it is above the 1.5 kg floor.
- Read the active ingredient on the box and confirm no pyrethroid is listed.
- Dose on the same calendar day every 30 days, and log the date.
- Treat every cat and the dog within 48 hours, then vacuum and wash bedding at 60 C.
- Comb the cat over a white sheet for 5 minutes every 7 days and count what you find.
A correct monthly dose costs a few dollars. A pyrethroid seizure costs a night in hospital and a cat that may not come home.
Frequently asked
My cat is 1.3 kg after a long illness. Can I use a quarter of a pipette?
Not safely. Splitting an ampoule gives an unknown volume, uneven spread and no safety data, and most spot-ons have no licensed dose below 1.5 kg. The usual options are a prescription oral flea product dosed by exact weight, a prescription spray, or mechanical control with environmental treatment until the cat is back above 1.5 kg. Ask the vet which of the 3 fits this cat's kidney and liver results.
The cat is indoor only. Does it still need a monthly dose?
Yes, in a house with a dog, with other cats that go out, or with people who walk in from a garden. Fleas arrive on shoes and clothing and on visiting animals, and a single female can start an infestation in carpet within 21 to 28 days. Indoor status lowers the risk, but it does not remove it, and an untreated indoor cat is the reservoir that keeps a treated cat reinfested.
What do I do if I accidentally used a dog product on the cat?
Wash the cat with mild dish soap and lukewarm water right away, without scrubbing the skin, then call the clinic or a poison line and expect to go in. Signs usually start 2 to 12 hours after contact and include ear twitching, drooling, tremors and fever above 40 C. Do not wait for symptoms to appear, and do not give any other flea product until the vet clears it.