Red man syndrome (RMS) is a potential adverse reaction related to vancomycin administration. Symptoms include flushing, erythema, and pruritus of the upper body as well as chest or back pains and hypotension. Differential diagnosis includes anaphylaxis, which features wheezing, dyspnea, and angioedema. RMS is caused by vancomycin activating mast cells. Risk factors include certain antibiotics, opioids, plasma expanders, and mastocytosis. RMS can be prevented by administering vancomycin at concentrations less than 5 mg/ml at rates less than 10 mg/min, and premedicating with diphenhydramine and famotidine. Treatment involves