For what reasons shouldn't you eat before sedation?

Sedation: To avoid vomiting & aspiration, which rarely happens from sedatives.

Side effect: As of 2014 the general guidelines for IV sedation dentistry are as follow: no solid food for six (6) hours prior to the procedure. All patients may have clear liquids until two (2) hours before the procedure. Diabetic patients will get special instructions as far as food intake is concerned. This is due to possible postsedation rare (less than 1%) side effect of nausea & vomiting.

Pulmonary aspiration: Since you are unable to control your gag reflex, if you eat something before surgery your stomach contents could regurgitate into your lungs.