Gentamicin × tacrolimus

Additive nephrotoxicity via two distinct mechanisms. Gentamicin (an aminoglycoside) directly injures proximal renal tubules. Tacrolimus (calcineurin inhibitor) causes afferent arteriolar vasoconstriction. Co-administration sharply raises acute kidney injury risk, especially in transplant patients.

Reduced urine output, rising creatinine and urea. In kidney transplant patients: accelerated graft decline. Gentamicin ototoxicity too: tinnitus, hearing loss.

Avoid the combination. Alternative antibiotics: a cephalosporin, carbapenem, or vancomycin (with its own renal monitoring). If gentamicin is irreplaceable: minimum effective dose, daily creatinine checks, and gentamicin level monitoring per protocol.