|MadSci Network: Medicine|
The most frequent cause of weight gain in older athletes is caloric imbalance--they tend to be less active and to continue to consume calories at a high level. This results in weight gain. It may be exacerbated if they were taking an anabolic steroid which allowed them to consume a large number of calories in order to gain weight and they failed to adjust their calorie intake downward when they quit using the drug. I'm not familiar with any particular impact of drug use on weight gain. It is important to recognize that the long-term health impact of drug use is very negative and these individuals are at risk of heart disease, renal disease, and liver disease.
Try the links in the MadSci Library for more information on Medicine.