If you are currently being treated with any of the following medications, you should not use creatine without first talking to your healthcare provider.

Cimetidine, Diuretics, NSAIDs, and Probenecid
Creatine may increase the likelihood of damage to the kidneys if taken with cimetidine (a medication for used for heartburn and to prevent ulcers), diuretics, probenecid (used for gout), or non-steroidal anti-inflammatory (NSAID) medications (such as ibuprofen).

Other substances
The combined use of caffeine, ephedra (a very concerning substance frequently found in sports or weight loss supplements), and creatine has been associated with at least one report of a stroke in a male weight lifter. It is believed that the combination of creatine with herbs and medications that stimulate the central nervous system contributed to the stroke.