Consult the CAA Medical Branch regarding prescriptions, they're very helpful and will always advise you and make alternative suggestions if necessary.
As for your "coach" at the gym, disregard his "advice" (
) and never take any supplements, you don't need them regardless of what he/she may say, unless your GP recommends them.
Supplements sold by gyms are essentially a significant addition to the owners income, on average the mark up is around 500%.
As an ordinary non professional sports person you simply don't need them, they do nothing for you that a bit of extra hard work on the equipment won't do for you, never use them unless advised to do so by your GP or better qualified person (rarely, if ever, found in gyms).
Basically, it's crap in, crap out.