Kitten Deworming: Minimum Ages and 0.5 kg Errors

Dewormers are sold by weight and licensed by age, and a kitten fails on the smaller of the two numbers. The four drugs most often used in kittens have minimum ages of 2 weeks for praziquantel, 3 weeks for fenbendazole, 6 weeks for selamectin, and 8 weeks for ivermectin. Under those ages the risk is not a weaker effect; it is toxicity in a 300-500 g animal with almost no fat reserve.

Four Drugs, Four Minimum Ages

Praziquantel at 2 weeks covers tapeworms and is the earliest of the group; many US labels print 6 weeks, so the 2-week figure applies only to products whose label states it, and a vet should confirm which one you hold. Fenbendazole at 3 weeks covers roundworm, hookworm, and whipworm, dosed at 50 mg/kg daily for 3 days. Selamectin at 6 weeks is a topical spot-on at 6 mg/kg that also handles fleas, ear mites, and sarcoptic mange; some labels say 8 weeks. Ivermectin at 8 weeks, at 0.2-0.4 mg/kg, is the last to clear because the margin between a therapeutic and a toxic dose is the narrowest of the four.

Dose by Weight First, Age Second

Every product has 2 gates: a minimum age and a minimum weight, often 0.5 kg. A 6-week-old at 0.45 kg fails the weight gate even though it passes the age gate, and splitting a tablet does not fix that. Weigh on a 5 g scale (the kitten weight tool takes the number in grams), use the exact figure, and choose the product whose label matches both gates. Kittens between 0.5 and 1.0 kg sit in the range where a 1 mL syringe with 0.01 mL marks is the right tool; a 3 mL syringe cannot deliver 0.18 mL accurately.

Where the 0.5 kg Rounding Goes Wrong

Most owners dose in 0.5 kg steps because that is how the chart reads. Take a 0.62 kg kitten on a 10 mg/kg product at 50 mg/mL: the correct volume is 0.12 mL, the 0.5 kg step gives 0.10 mL (17% under), and rounding up to 1.0 kg gives 0.20 mL (67% over). At the other end, a 1.4 kg kitten rounded to 1.5 kg is a 7% error, which is harmless. The same 0.5 kg step is 7% at 1.4 kg and 67% at 0.62 kg; the error scales inversely with body weight, which is why small kittens are the ones hurt by it.

Age, Weight, Dose Table

Drug	Min age	Min weight	Dose	Repeat	Course
Praziquantel	2 weeks	0.5 kg	5 mg/kg	every 2 weeks	3 doses
Fenbendazole	3 weeks	0.5 kg	50 mg/kg	daily	3 days
Pyrantel pamoate	2 weeks	0.45 kg	5 mg/kg	every 2 weeks	4-6 doses
Selamectin	6 weeks	0.5 kg	6 mg/kg	monthly	3 months
Ivermectin	8 weeks	0.5 kg	0.2-0.4 mg/kg	every 2 weeks	2-3 doses

The Real Schedule: Weeks 2, 4, 6, 8, 10, 12

Roundworm larvae reach kittens through milk, so the clock starts before birth. The standard plan is a dose at 2, 4, 6, 8, 10, and 12 weeks, then monthly to 6 months, then 3-4 times a year for an indoor cat and monthly for a cat that hunts or goes outside. Six doses in the first 12 weeks is not over-treatment; it matches a 2-week prepatent period in a kitten that is reinfected daily from the queen and the environment.

Why the Interval Is 2 Weeks and Not 1

Pyrantel and praziquantel act on the adult worm in the gut and are not stored, so efficacy at day 14 is near zero against larvae that were encysted on day 1. A 7-day interval adds doses without adding coverage, and each dose is another chance for a 0.4 kg kitten to vomit or drool for 20 minutes. Fenbendazole is the exception: it is given 3 days in a row because it works gradually on the worm's tubulin, not in a single hit.

Products That Can Kill a Kitten

Permethrin, found in many dog spot-ons at 45-65%, causes tremors, hypersalivation, and seizures in cats within 1-4 hours and is a genuine emergency at any dose. Never split a dog pipette, never use a dog shampoo, and never use human head-lice shampoo (permethrin 1%) on a kitten. Essential-oil flea drops with tea tree or pennyroyal are also unsafe. Read the label for the species and the age every single time, even on a product you used last year.

What to Watch for 6-24 Hours

Brief drooling or 1 episode of soft stool in the first 2-6 hours is common with pyrantel and usually passes. Not common: vomiting more than 2 times, wobbling, dilated pupils, tremors, or a kitten that stops eating for 24 hours. Those signs after ivermectin or a spot-on mean a call to the clinic or poison control immediately. Keep the box with the lot number; it is the first thing the vet asks for. Flea-product safety overlaps heavily with deworming, and the age limits per product are in kitten flea treatment safety.

When to Call the Vet Instead of Dosing

Do not dose at home and instead book an appointment for: a kitten under 3 weeks, a body weight under 0.45 kg, diarrhea lasting more than 24 hours, a distended belly with poor weight gain, worms visible in stool or vomit after 2 rounds, a queen who is pregnant or nursing (most products are off-label), or any kitten that has reacted to a dewormer before. Maternal antibody and vaccine timing also interact with the deworming schedule, which is why the two plans are built together in maternal antibody and vaccine timing.

Five Steps Before the Next Dose

• Weigh on a 5 g scale and dose the exact weight, not the nearest 0.5 kg.
• Check both gates on the label: minimum age and minimum weight.
• Use a 1 mL syringe with 0.01 mL marks for any kitten under 1 kg.
• Repeat every 2 weeks through 12 weeks, then monthly to 6 months.
• Watch for 6-24 hours and keep the packaging with the lot number.

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