Which antibiotic is often used to treat MSSA infections in an intravenous form?

Nafcillin is the antibiotic of choice for treating infections caused by methicillin-sensitive Staphylococcus aureus (MSSA) when an intravenous formulation is required. Nafcillin is a penicillinase-resistant penicillin that is specifically effective against MSSA due to its structure that allows it to evade degradation by β-lactamase enzymes commonly produced by these bacteria.

Nafcillin is administered intravenously in cases of severe infections, such as cellulitis, abscesses, pneumonia, endocarditis, and osteomyelitis that are complicated by MSSA. Its pharmacokinetics and spectrum of activity make it particularly suitable for serious infections where prompt treatment is necessary.

Other antibiotics listed, such as doxycycline, metronidazole, and clarithromycin, have different mechanisms of action and spectrums of activity. Doxycycline is more useful against atypical bacteria and certain other pathogens but is not the frontline treatment for MSSA specifically. Metronidazole is primarily used for anaerobic infections and protozoal infections, while clarithromycin is macrolide and is more effective against respiratory pathogens. Therefore, while these alternatives can be useful in other contexts, nafcillin's specific efficacy against MSSA makes it the