S-mephenytoin 4-hydroxylation in older Americans.

To examine whether a drug-metabolizing enzyme changes with normal aging, the S:R index of S-mephenytoin 4-hydroxylation was determined in 150, unmedicated elderly Americans (mean age 75.4). Ten (6.7%) were identified as categorically slow metabolizers (S:R ratios greater than or equal to .95). This increased incidence of slow metabolizers was accounted for by a significant and previously unreported, increased proportion of slow metabolizers among the black (18.5%) as compared to the white subjects (4.1%) (P = .017). There was no relationship found between S:R ratios and age or creatinine clearance.