[After bariatric surgery effects of medication are not always predictable].

Bariatric surgery is widely used as an effective treatment for obesity. Changes in the anatomy of the digestive tract as a result of these operations may lead to changes in drug availability. This is illustrated by three cases in which problems arose with calcium metabolism, thyroid hormone substitution or weight. Effects on medication after bariatric surgery are not always predictable and there are no uniform guidelines how to handle. We advocate therefore a proactive attitude of physicians by monitoring direct effects (such as blood pressure, glucose, or mood) or determining drug concentrations, to prevent treatment failure or toxicity.