A therapeutic dosage of methamphetamine (Desoxyn) is about 5mg - 15mg, whereas a recreational dosage can be as high as several hundred milligrams. Though certainly toxic at higher doses, research hasn't shown that the drug is toxic at therapeutic levels. Like the majority of drugs, doctors must balance its benefits with any potential side effects.

You can disagree with my comparison if you'd like, but the fact remains. Don't take my word for it - I encourage you to read some of the research out there surrounding therapeutic methamphetamine as well as the more common levo/detroamphetamine.