R-5-(4′-hydroxyphenyl)-5-phenylhydantoin (R-HPPH) is associated with a higher incidence of gingival hyperplasia (OR: 1.0; 95% CI: 1.00–1.03) and PHT (OR: 1.09; 95% CI: 1.00–1.19).R-HPPH (73.92 ± 48.14 ng/mL; p = 0.03) was lower in CYP2C19 poor metabolizers (OR: 0.76; 95% CI: 0.22–2.66; p = 0.67) and CYP2C9 poor metabolizers (OR: 0.44; 95% CI: 0.11–1.78; p = 0.25), but no association was found between genotype and gingival hyperplasia. The gene discussed is CYP2C19; the disease is pulmonary hypertension, primary, 1.