Q. Are there drugs that turn into toxic substances as they age?

A. Though the main risks of using outdated medications lie elsewhere, a rare kidney ailment called renal tubular acidosis has been reported to result from taking the antibiotic tetracycline old enough to have become degraded into other chemicals.

The initial case report, involving a form of the drug that is no longer used, was published in Annals of Internal Medicine in 1963.

There were a handful of subsequent incidents, leading to a longstanding warning about taking that drug when it is outdated.

A 2004 review of the literature on tetracycline and similar antimicrobials suggested that it might be difficult to determine when the kidney ailment is caused by such a drug.