Fipronil ± (S)-methoprene

What is this medication for?

• Fleas, ticks, chewing lice (label); extra-label successes: chiggers (Trombicula), sarcoptic mange, cheyletiellosis, otoacariosis

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Temporary application-site irritation; wet/oily appearance ≤24 h (Common, minor)

Worth a phone call to your veterinarian

• Hypersensitivity (Rare)
• Permethrin confusion-alert (Fipronil CAT-safe; K9 Advantix (permethrin) is the dog-only product — permethrin is highly toxic to cats)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Avoid human contact until dry; kittens/puppies per label age minimums

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Other topicals at same site

Storage and handling

Applicator tubes; EPA-registered — pesticide storage/disposal rules apply; keep off skin until dry.

Good to know

• Pharmacy-law note: Frontline = EPA pesticide (OTC), Comfortis = FDA drug (Rx) — same shelf-space problem, different regulatory worlds.
• Mechanism trio for the exam: fipronil = GABA-chloride, imidacloprid = nicotinic-ACh, permethrin = sodium-channel.
• The oils-and-follicles depot explains both the 24-hour spread and the residual — and why shampooing within 48 h undermines the application.