When taken under a doctor's supervision, fast-weight-gain tablets can be safe. However, some side effects can occur, depending on the active ingredient in the tablet, says Drugs.com.
Methyltestosterone boosts the production of testosterone and should be taken with food to avoid an upset stomach, explains Drugs.com. A doctor may also prescribe Oxandrolone, which is an anabolic steroid designed to regain lost weight following a severe illness or surgery; it's best to make this tablet part of a balanced diet to avoid constipation.
A study conducted at University of Essen, published on the NCBI's PubMed.gov website, concluded that Oxymetholone can also safely gain back weight; patients taking this tablet gained 14 percent of their starting weights after two months.