Are certain classes of drugs more often implicated in creating problems with respect to interactions or tolerability?
That is an important question because it really focuses on the importance of understanding the pharmacology of each drug. Psychotropic drugs in each class are sometimes considered to be equivalent. Two very widely used SSRIs, however, paroxetine and citalopram, have very different mechanisms of metabolism. Paroxetine is particularly interesting, because of all of the SSRIs, it has the most exclusive pattern of metabolism. For the average person with adequate CYP 2D6 enzymes, almost the entire metabolism of paroxetine is through the 2D6 pathway. Approximately 10% of our patients who are of European ancestry do not have adequate levels of the CYP 2D6 enzyme, because they have a genetic defect in one or both of their 2D6 genes. Consequently, they are unable to tolerate standard doses of paroxetine. In contrast, patients with an inadequate CYP 2D6 enzyme, but who have adequate CYP 2C19 enzymes, would have no trouble metabolizing citalopram. The adequacy of both the 2D6 and 2C19 enzymes pro
