When discussing PGx, we classify a person according to his/her phenotype (their metabolic capacity for a given enzyme).
A poor metabolizer (PM) is a person who lacks a fully-functional enzyme or has reduced expression of the enzyme and therefore exhibits decreased metabolism of drugs. This person would require lower doses of a drug that is metabolized by that enzyme. A PM who receives a standard dose is more likely to experience unwanted side effects or toxicity. A PM can also experience diminished effects with drugs that need to be metabolized to active compounds by the enzyme in question.
An intermediate metabolizer (IM) has one wild-type (normal) copy of the gene and one absent or dysfunctional copy. The IM group is very heterogeneous.
A person with normal enzyme activity is referred to as wild type or as an extensive metabolizer (EM). This person should respond to standard dosages of a drug. Most people are EMs. This is the population in which most dosing regimens have been worked out in clinical trials.
An ultrarapid metabolizer (UM) will require a higher dose than usual since he/she has a polymorphism (mutation) that codes for a more efficient enzyme or more expression of an enzyme. They will eliminate the drug more quickly. A UM may be resistant to standard treatments, and it may take some time to adjust the dosage before therapy is achieved.