Diet pills in general are unhealthy. Using them while underweight can be dangerous. I would suggest against it. Most diet pills that you can purchase at retail stores up your metabolism by a special blend of herbs and drugs (such as caffeine) to raise your core body temperature and increase your heart rate. Most do not block the absorption of fat.
If you feel you need to hide them then perhaps you should listen to your conscience: there may be something wrong with the idea. Any unhealthy weight-loss plan could be considered an
ED behavior.