– Why are certain animal medications given by weight, even if the animal is a tiny bit over the limit?

Question

For example: Most flea medications range from 5-20 pounds, 21-55 pounds, and 56+ pounds. Why would a pet who is 22 pounds require a dose that is also suitable for a 55 pound pet? Isn’t there a concern of over medicating since there’s only a 2 pound difference?

In: Chemistry

Answer 1

This is reflecting something known as an ld50, where its measured in a weight per weight ratio where its toxic. Say a drug has an ld50 of 10 mg/kg, a 100 kg man would need to have 1000 mg or 1g of that drug for it to likely kill them (if we had 10 100 kg participants, 5 would likely die (hence ld50)). Likewise for doggos.