Midazolam (major): CYP3A4 inhibition. Olkkola 1993 (PMID:8453848): erythromycin 500 mg TID × 7 d raised oral midazolam AUC >4× and reduced IV midazolam CL by 54% (n=12 healthy). Avoid; choose azithromycin if a macrolide is needed.
Triazolam (major): CYP3A4 inhibition. Greenblatt 1998 (PMID:9757151) same 5-arm study: oral CL of triazolam 413 → 146 mL/min with erythromycin → AUC ratio 2.83. Ki ≈ 3/(2.83−1) = 1.64 µM. Cross-check vs erythromycin→sildenafil Ki 1.67 — internally consistent.
Sildenafil (warn): CYP3A4 inhibition. Muirhead 2002 (PMID:11879258) n=26 male, erythromycin 500 mg BID × 5d, single 100 mg sildenafil: "AUC and Cmax of sildenafil (2.8-fold and 2.6-fold, respectively)". AUC_R = 2.8 → Ki ≈ 3/(2.8−1) = 1.67 µM. Pfizer recommended starting at 25 mg sildenafil with erythromycin.
Alfentanil (major): CYP3A4 inhibition (anesthesia DDI). Bartkowski 1989 (PMID:2501060) n=6 IV alfentanil ± erythromycin × 7d: t½ 84.0→131.4 min, CL 3.9→2.9 mL/kg/min → ratio 1.34. Ki ≈ 3/(1.34−1) = 8.8 µM. NB: IV bypasses first-pass CYP3A; hepatic-only Ki here is higher than the oral-derived value. Authors recommend reduced alfentanil dose or avoidance with erythromycin.