We assume that all your P-drugs have already been selected on the basis of efficacy. However, you should now verify that the drug will also be effective in this individual patient. For this purpose you have to review whether the active substance is likely to achieve the therapeutic objective, and whether the dosage form is convenient for the patient.
Convenience contributes to patient adherence to the treatment, and therefore to effectiveness.
Complicated dosage forms or packages and special storage requirements can be major obstacles for some patients. Contraindications are determined by the mechanism of action of the drug and the characteristics of the individual patient. Drugs in the same group usually have the same contraindications. Some side effects are serious for categories of patients only, such as drowsiness for drivers. Fortunately, in practice only a few interactions are clinically relevant.