Why Does A Doctor Want To Test You For Meth

Why does a doctor want to test you for meth etc. & how can it have an effect when it comes up positive on meth & you have neen diagnosed for a d d. Also psycologist had given u a rx for amphetimines due to a d d & also to give u a bump start due to severe deppresion to het u motivated but no longer follow up on th 3 month check- ups for th 3 part rx?