Listen love, forget all about OTC drugs
and do this the healthy way. A successful weight loss plan is not one that will enable you to lose the most weight the fastest, it's one that can be incorporated into your day to day routine, enabling you to get down to your desired weight...and remain there.
Sleeping for two days (that would need a sledgehammer, not an OTC antihistamine
) will not shrink your stomach sufficiently to cause a loss of appetite. As has already been said, it will just push your body into a kinda "siege mode"
slowing your metabolism down in it's attempt to retain calories. You'll also retain water, which will not only be unhealthy, it will also add to your weight.
I can't understand how any qualified trainer would advise you to starve yourself to lose weight, because not only is it extremely unhealthy, it could also be quite dangerous. One of the healthiest ways you can lose weight with very little effort (as has already been said) is by eating smaller portions, taking the stairs rather than using the elevator, walk (where possible) rather than drive.
But that does take time.
Try taking some exercise maybe three to four times a week, you'll find that afterwards you'll have very little appetite. This is due to the bodies own natural opioid
called endorphins. These will also make you feel good, raise your energy levels, and they have a positive effect on your mood too.
But one thing that is really important...always consult a GP (doctor) just to make sure there are no underlying medical reasons why you've gained weight in the first place. Also check out that it's ok for you to exercise and diet, it might be necessary for you to avoid certain exercises. They'll also be able to advise you as to a healthy fitness/diet regime. One that will get you slim and more importantly, keep you that way.
Take care and stay safe love.