Studies have shown metronidazole to be useful in the treatment of colonic Crohn's disease, especially for perineal complications. It appears to equal sulfasalazine in effectiveness and to offer hope to patients who fail to respond to that drug. However, exacerbation of perineal disease is to be expected on discontinuation of therapy. The only troublesome side effect is paresthesia, which apparently is dose-related and not progressive. Although no studies exist showing the drug to be mutagenic, teratogenic, or carcinogenic in humans, animal and laboratory tests raise concern of this possibility. For this reason, metronidazole therapy should be stopped in two to six months if ineffective and stopped periodically when disease is controlled to see if it can be discontinued.