When your cat suffers from respiratory disease, drugs to treat it usually come in different forms, for different administration ways : oral, inhaled, or injected. Orally administered or injected drugs need to travel through the body before reaching the organ to treat (in this case, the lungs). This is called systemic treatment.
Inhaled drugs are directly administered to the lungs. Thus, they do not reach other organs and more of the administered drug dose is available for the lungs. This reduces both the amount of drugs to be administered and the risk of side effects. Moreover, because inhaled drugs are administered directly to the area to be treated, their action is very fast, making them suitable for treating asthma crisis without the need for chronic treatment.
By contrast, systemic drugs have to travel through the body to reach the lungs. Thus, most of the drugs reach organs they are not intended for and only a small proportion of the administered drugs reaches the lungs. This is why the amount of drugs to administer has to be increased to reach the same effect as the inhaled drugs. Both the fact that drugs reach organs they are not intended for and the increased amount of drugs to administer increase the risk of side effects. In addition, their action is slower, so they can only be used for chronic treatment.
Furthermore, inhaled treatment may be easier to administer, particularly if the advice to get your cat used to the mask have been followed.