Variables other than mutations also affect CYP450 enzymes. Many drugs are able to induce CYP450 enzymes, and CYP450s can be inhibited by a variety of substances. For example, CYP2D6 can be inhibited by the common medications cimetidine (Tagamet) and fluoxetine (Prozac).

Since many patients are on multiple medications and since dietary and environmental factors can change, CYP450 expression levels cannot be solely predicted based on their genotype.

Some CYP450 inducers and inhibitors are listed in the table on the following page.