Comfortis tablets are given once a month to control fleas on dogs and cats. The drug is lethal to blood sucking insects but has little or no effect on dogs. (Occasionally dogs will vomit after taking a tablet.) It is best to give Comfortis with food. This reduces the chances of vomiting as well as enabling a more persistent flea-killing effect afterwards, so that one tablet will last for the whole month. Comfrotis tablets come in five different sizes to suit dogs of different body weights and two sizes for cats. Comfortis is not recommended for dogs under 14 weeks of age, or less than 3.9kg bodyweight.