Digestive Problems
Numerous diet pills work by blocking or reducing fat absorption. Since these drugs interact with your digestive processes, you may experience mild to severe digestive symptoms. Xenical and Alli -- a milder version of Xenical available without a prescription -- for example, may cause anal discharge, gas, frequent bowel movement and/or diarrhea. If you consume greasy or high-fat foods while taking the pills, your chance for developing digestive problems is heightened, according to MayoClinic.com. Abdominal cramps, lethargy and dehydration may also occur, particularly if your diarrhea symptoms are severe. Prescription weight loss pills, such as zonisamide, may cause nausea.