Which antibiotic should be avoided in a patient with a history of sulfa drug allergy?

The choice to avoid trimethoprim-sulfamethoxazole, commonly known as Bactrim, in a patient with a history of sulfa drug allergy is rooted in the medication's composition. Trimethoprim-sulfamethoxazole is a combination antibiotic that includes sulfamethoxazole, which is a sulfonamide. Patients with a sulfa allergy may experience allergic reactions when exposed to sulfonamide antibiotics, leading to potentially serious side effects such as rashes, fever, and even anaphylaxis. Therefore, for someone with a known sulfa allergy, this medication poses a significant risk.

In contrast, the other antibiotics mentioned do not contain sulfonamides and are not structurally similar to sulfa drugs. Cephalexin, for example, is a cephalosporin antibiotic that does not cross-react with sulfonamide allergies. Ampicillin is a penicillin derivative, and nitrofurantoin is an antibacterial agent with a different mechanism of action. As these alternatives do not contain sulfa components, they can be safer options for patients with sulfa allergies.