Yes, they can: the doctor is allowed to exercise his/her medical jugment in terms of setting, changing, or reducing prescriptions. There is no way to stop them from doing this before hand; IF as a result of it, you suffer a serious injury, impairment, etc. afterwards, you may be able to sue for malpractice, but you have to wait and see if you are harmed (and if so, how much--lesser amounts of harm are not worth the cost of a malpractrice lawsuit). But you can't substitute your judgment for the doctor's; if he/she thinks it is appropriate to reduce your prescription, they can do so.