Complications with anesthesia are very rare, but the most common cause is patient error. Your anesthesiologist or surgeon should provide you with detailed instructions on what to do and what not to do before receiving anesthesia. One of the most important things that almost all anesthesiology patients need to do is fast for several hours before their surgery. If your stomach is full of acid or food contents, it can regurgitate back into the esophagus or lungs, which can lead to serious health problems.