Of course, there are other causes of cats hair loss. Cats can have a type of obsessive/compulsive disorder and lick themselves until their skin is raw. Sedatives and antidepressants along with Feliway can help such disorders, but there is no doubt in my mind that this is not the problem that Whiskers has. Also, the open sore on his lip is almost definitely not from his licking, but is another allergic reaction to the fleas, often referred to as Rodent Ulcer (eosinophilic ulcer). The steroids will help with that condition also.
Steroids can be injected or taken orally so that they circulate and influence the entire body—systemic steroids—or they can be used topically. Topical steroids are safer than injected or oral steroids because so little is absorbed that the possibilities of side effects are minimal. Examples of prescription systemic steroids include Prednisone and Methylprednisolone . Examples of prescription topical steroids include eye drops such as Prednisolone Acetate Ophthalmic Suspension (for dogs), hair conditioner (ResiCort®), or Genesis Topical Spray (for dogs).