Drugs are selected based on characteristics of the drug (eg, efficacy, safety profile, route of administration, route of elimination, dosing frequency, cost) and of the patient (eg, age, sex, other medical problems, likelihood of pregnancy, ethnicity, and other genetic determinants). Other drugs that the patient may be taking can also influence the physician's drug selection. Risks and benefits of the drug are also assessed; every drug poses some risk.
Adherence (compliance) is the degree to which a patient follows a treatment regimen. For drugs, adherence requires that the prescription be obtained promptly and the drug be taken as prescribed in terms of dose, dosing interval, duration of treatment, and any additional special instructions (eg, taking the drug without food). Patients should be told to alert their physician if they stop or alter the way they take a drug but they rarely do so.
Pharmacodynamics (sometimes described as what a drug does to the body) is the study of the biochemical, physiologic, and molecular effects of drugs on the body and involves receptor binding (including receptor sensitivity), postreceptor effects, and chemical interactions. Pharmacodynamics, with pharmacokinetics (what the body does to a drug, or the fate of a drug within the body), helps explain the relationship between the dose and response, ie, the drug's effects. The pharmacologic response depends on the drug binding to its target. The concentration of the drug at the receptor site influences the drug’s effect.
Pharmacokinetics, sometimes described as what the body does to a drug, refers to the movement of drug into, through, and out of the body—the time course of its absorption, bioavailability, distribution, metabolism, and excretion.