Flea treatment after collar reaction in cats

Hair loss around the neck from a discount flea collar is a well-known reaction — those cheap collars often contain older chemical compounds at poorly regulated concentrations, and cats in particular are very sensitive to them. Before we find something safer for your Bombay, I want to flag one critical point: many insecticides that are fine for dogs are outright toxic to cats, so using the wrong product can be fatal — always essential to check anything is cat-specific. Can you tell me roughly how much your cat weighs, whether you're dealing with an active infestation right now or trying to prevent one, and have you treated your home as well?

That's a really common collar failure pattern — the active ingredient front-loads and fades fast, and the irritation often shows up just as the efficacy is dropping. At 4kg your Bombay is a healthy adult weight and she's straightforward to treat. With live fleas still on her, you need something that kills fast. What's her current health like — any known skin conditions, and is she an indoor-only cat or does she go outside?

Perfect — that makes the decision easy. I'd go with Advantage spot-on for cats, specifically the 40mg pipettes for cats 4kg and under (your cat sits right at the boundary, so double-check the pack, but there's also an 80mg version for cats over 4kg if you want the higher dose confirmed by your vet). Advantage uses imidacloprid, which is cat-safe, kills adult fleas on contact within 24 hours without needing a bite, and has a solid track record on sensitive cats. It's applied to the back of the neck where she can't groom it off. One pipette lasts a month. Because she's going into the garden you'll want to stay on a monthly schedule through the warmer months. You'll also need to treat the house — around 95% of a flea infestation lives in the environment, not on the cat. A household spray like Indorex or Acclaim is the standard recommendation for that.

Advantage is genuinely fast — you should see fleas dying and dropping off within a few hours of application, with near-complete kill within 24 hours. Frontline uses fipronil, which is also cat-safe, but it works differently and requires the flea to bite, which is a slower process. For a cat that's already being bitten and is likely uncomfortable, Advantage's contact kill is a real advantage right now. Frontline is a reasonable long-term option too, but for knocking down an active infestation on a cat who just had a skin reaction, I'd stick with Advantage. Once the infestation is cleared and you've treated the home, you can reassess whether to continue with it or switch.

Good instinct to ask, but no — the spot-on goes on the back of the neck at the base of the skull, on the skin, not on the fur. The hair loss from the collar would typically be lower down where the collar sat. If the skin at the application site looks raw, inflamed or broken, I'd hold off a day or two and let it settle, or apply slightly further back between the shoulder blades as an alternative site. If the irritation is significant or she's been scratching hard at it, a quick vet check wouldn't hurt before you apply anything. But if the skin looks basically normal just with thinner fur, you're fine to treat straight away.