Most spot-on failures are application failures, not product failures. The tube goes onto the fur instead of the skin, or down the spine where the cat can reach, and 3 weeks later there are still fleas. Done right, one tube on the nape at the skin covers 30 days.
Four numbers do the work: the tip touches skin, the dose matches the weight band, no bath for 48 hours either side, and 2 hours of separation in a multi-cat house.
Why the nape and not the spine
The base of the neck between the shoulder blades is the one 3 by 5 cm patch a cat cannot reach with its tongue. The drug spreads through the sebum layer over 12 to 24 hours, so it needs skin contact at the start, not fur contact. A line down the spine looks tidy and puts the drug exactly where a grooming cat can lick it, which is how you get drooling and tremors instead of flea control.
The application, in 4 steps
Open the tube upright and keep the tip up so it does not leak. Part the fur with a finger or the tube cap in 3 strokes until you see skin, making a 1 cm wide parting just in front of the shoulder blades. Place the tip flat against the skin, not hovering 5 mm above it, and squeeze steadily while drawing the tip back 2 to 3 cm, so the liquid lands in a short line on skin. Close the parting with 2 fingers, do not rub it in, and keep the cat from grooming for 5 minutes while it dries.
Long-haired cats need the fur parted twice, and a 2 cm clip with scissors at the application site is acceptable and grows back in 3 to 4 weeks. Two people help: one holds a 2 kg cat still for 20 seconds, the other parts and squeezes.
Dose by weight band
Never split one tube between 2 cats to save money. The volume is matched to body weight, and a half dose on a 5 kg cat leaves 2 to 3 weeks of sub-lethal exposure, which is how resistance starts. Use the band on the label:
| Cat weight | Tube volume | Active load | mg per kg | Repeat |
|---|---|---|---|---|
| 1.0-2.5 kg | 0.25-0.3 ml | 15 mg selamectin or 25 mg fipronil | 6-10 | Every 30 days |
| 2.6-5.0 kg | 0.5 ml | 30 mg or 50 mg | 6-12 | Every 30 days |
| 5.1-8.0 kg | 0.75-1.0 ml | 45-60 mg or 75-100 mg | 6-12 | Every 30 days |
| Over 8 kg | 1.0 ml, vet-adjusted | Vet-scaled | 6-10 | Every 30 days |
An 8 kg cat is not a 2 tube cat by default; some labels cap at 7.5 kg and require the vet to choose. A kitten under 8 weeks or under 1 kg usually gets no spot-on at all, only a flea comb and a bath with warm water and mild soap, because the surface area to body weight ratio makes toxicity likely.
The 48 hour bath rule
Bathing 48 hours before application strips some of the sebum the drug binds to, and bathing 48 hours after washes it off before it has distributed. Most products are water-resistant after 24 to 48 hours, which means a cat that walks through a puddle on day 3 is fine and a cat bathed on day 1 is not protected. If a bath is unavoidable, use a shampoo labelled for use with spot-on products and wait 72 hours.
Swimming, rain and a wet towel are not the same as a bath, but a cat that gets soaked to the skin within 24 hours should be checked for fleas at day 10 rather than assumed covered.
Multi-cat house: 2 hours apart
Two cats will groom each other's necks within minutes. Separate them for 2 hours, or until the application site is dry to the touch, and apply at different times of day so one is not still wet when the other is released. A cat that licks a fresh dose drools, may vomit once, and can show fine tremors for 1 to 2 hours.
What to do if that happens: offer water, keep the room quiet, note the time, and call the vet if tremors last more than 30 minutes or the cat cannot walk straight. Do not bathe the cat to wash it off unless the vet says so, because a wet, chilled cat that is already trembling is worse off.
Four mistakes that keep fleas alive
- On the fur, not the skin: the tube empties into the coat and never reaches the sebum, so 7 days later the cat is scratching again.
- Down the spine: reachable, lickable, and the reason for drooling and tremors.
- One tube, 2 cats: half doses select for resistant fleas and leave both cats uncovered.
- A dog product on a cat: permethrin at 45% or more causes tremors, hypersalivation and seizures in cats, and is a veterinary emergency within 1 to 4 hours.
Also skip the re-apply reflex. If fleas are still visible on day 10, the answer is a second product class or an oral, not a second tube of the same drug a week early. The flea life cycle runs 3 weeks from egg to adult and pupae survive 6 months in carpet, so 30 day dosing for 3 consecutive months is what breaks a household infestation. Details on kitten-safe products are in kitten flea treatment safety, and the September to November surge is covered in the autumn second flea wave.
Indoor cats and winter
Indoor cats get fleas from a dog, a human's trousers, or a visit to the carrier. Keep 12 months of cover in a house with a dog, and 12 months in a warm flat where the heating keeps the floor at 20 Celsius or more. In a cold house with no other animals, stopping for 2 to 3 winter months is a reasonable choice, but restart 4 weeks before the first warm week. Note the dates on the vaccine calendar so the annual exam and the flea plan land together.
Five steps on application day
- Weigh the cat, pick the tube from the label band, and check the expiry date.
- Part the fur 3 strokes to skin, 1 cm wide, between the shoulder blades.
- Tip flat on skin, squeeze while drawing back 2 to 3 cm, then close the fur.
- Separate cats for 2 hours and hold off baths for 48 hours either side.
- Write the date on the calendar and recheck the coat at day 10, not day 3.
Frequently asked
What happens if my cat licks the spot-on treatment?
Expect drooling, one vomit and possibly fine tremors for 1 to 2 hours, because the carrier solvent tastes bitter. Offer water, keep the room quiet and note the time. Call the vet if tremors last beyond 30 minutes or the cat cannot walk straight, and never give a second product to settle it.
Can I apply flea treatment right after a bath?
Wait 48 hours. Bathing strips the sebum the drug binds to and distributes through, so a dose applied the same day sits on a dry coat and spreads poorly. The same 48 hour window applies after dosing, since most products only become water-resistant 24 to 48 hours later.
Why are fleas still on my cat 10 days after treatment?
Live fleas at day 10 usually mean the tube went on fur rather than skin, or that the environment is still producing new adults. The egg to adult cycle runs about 3 weeks and pupae survive up to 6 months in carpet, so keep 30 day dosing for 3 consecutive months and vacuum daily for the first 2 weeks.