Which antibiotic should not be prescribed to a patient taking simvastatin?

Erythromycin should not be prescribed to a patient taking simvastatin due to the potential for significant drug interactions that can increase the risk of statin-related side effects, particularly myopathy and rhabdomyolysis. Erythromycin is a strong inhibitor of the cytochrome P450 system, particularly the CYP3A4 enzyme, which is responsible for metabolizing simvastatin in the liver. When erythromycin inhibits this enzyme, it can lead to significantly higher levels of simvastatin in the bloodstream, increasing the risk of adverse effects.

In contrast, the other antibiotics mentioned do not have the same level of interaction with simvastatin. Ciprofloxacin, doxycycline, and vancomycin do not primarily use the CYP3A4 metabolic pathway and therefore do not pose the same risk when prescribed alongside simvastatin. Thus, while these alternatives may be appropriate for patients on simvastatin, erythromycin requires caution due to the potential for serious complications from the interaction.