While it might seem like an “easy” way to fix something you feel is wrong, there is actually no way to take a pill to grow or gain weight. If you feel that your height or weight is below what it should be, first talk to your health care provider to find out if you are growing normally – some people are naturally smaller or larger than their peers. By the age of 20 years old there is not much that you can do about your height (stature) because most people have finished growing by that age. However, if you are interested in gaining weight and your health care provider has indicated that it is safe to do so, you might consider meeting with a registered dietitian for tips such as increasing your dietary fat consumption (adding things like nuts, nut butters, seeds, oils, and avocado to your intake) and eating regularly throughout the day.
Also in the U.S. diet supplements to either lose or gain weight are not well regulated. This means they can have products added to them that can harm your body (including liver and kidney problems). Before trying any supplement it’s always a good idea to talk with your health care provider.