Lasix (also known as furosemide) is a diuretic (water pill) that is chemically related to a group known as "sulfa" drugs. The term sulfa refers to a specific chemical structure that contains a sulphur molecule. Other drugs such as sulfamethoxazole-trimethoprim (also known as Bactrim or Septra), some drugs used to treat diabetes, and hydrochlorothiazide also contain this sulfa structure.

People sometimes develop an allergy to the sulfa antibiotics. People can develop an allergy to other sulfa-containing drugs but it is much less common. To be safe, if a person is allergic to one sulfa drug, they should be considered allergic to them all. This is called cross reactivity.

Until recently, we really did not have any good data to guide us. The results of a study published in the New England Journal of Medicine last fall give us a good idea of the risks. The researchers looked at the records of almost 20,000 patients who had taken a sulfa-containing antibiotic. The study was a bit complicated, but the bottom line is that there is a risk of a reaction to drugs like Lasix in people who have had a reaction to drugs like Bactrim. The risk looks like it is increased in people with more than one drug allergy (for example allergic to penicillin and Bactrim) and those with asthma, hay fever and other allergies.