A U.S. soldier may have inadvertently discovered a loophole in consumer protection rules when he purchased fish antibiotics to treat his sinus infection, the New England Journal of Medicine reports.

Doctors at the Pentagon Clinic wrote in the journal Thursday that one of their patients admitted to buying penicillin and sulfa antibiotics "in the fish medication aisle" of a local pet store to treat his sinusitis.

Antibiotics sold for fish are not regulated for use in humans, the doctors said in the article. Even so, the soldier said the pet store was a common source of over-the-counter antibiotics, the doctors wrote.

Although prescriptions are required for most antibiotics for humans and pets, fish antibiotics appear to be the exception.

The doctors were able to find antibiotics such as erythromycin, penicillin, tetracycline and sulfathiazole at local pet superstores.

In Canada, antibiotics such as tetracycline and erythromycin are available online from pet stores such as SuperPet and Fins N Fur Pet Supplies. They are sold to treat such ailments as body slime and cottonmouth.

Some of the antibiotics carry the disclaimer "For aquarium use only."