Prescription meth use is much different than recreational use. Dosages are way larger, and so is method of administration. This makes enormous difference.

Consider dextrometorphan. This is a common over the counter cough medication. It can also be used recreationally - but the recreational dose is 30 times the therapeutic dose. Amphetamine is similar: methheads are not swallowing a single pill of Adderall, like medical users are.