Medicinal use of methamphetamine is very small in the USA. Heroin is legal for doctors to prescribe in Canada but is extremely rarely done so, while it is completely banned in the USA. Drugs like amphetamine and morphine are available in both countries and are prescribed pretty often.
What you're describing isn't the cause of the difference in violent crime. It's because the US makes being a drug trafficker an all-in proposition. Once you've broken those laws you really can't go back.
Consider a young guy who desperately needs money. Probably because he's meth addict. And thinks he's so clever to mail order $200 of meth and sell it to his sleazy friends. He does it a couple more times. Sells maybe $5000 in a couple years. If the police and prosecutors do it right, they can get him for multiple counts of meth trafficking on federal charges and he goes to prison for 20 years, period. No early parole federally in the US, you know.
A lot of aggressive young men would rather risk a shootout than go to prison that long. Given they're guilty of crimes punished as harshly as murder, why not commit murder and rob your rivals? What have you fucking got to lose? It just gets worse from there.
When the system works right in Canada, and honestly it often doesn't but it sometimes does, that young man gets 3 months in local jail, drug counselling, parole for a few years (violation like possession means back to jail for a month or two and more rehab) and then they try to find a job placement for him.
It's easier to go back to normal life here too. Arrest mugshots for such crimes don't make the papers. It's illegal to discriminate because of criminal record without reason in about half of the provinces (no drug addicts working in pharmacies, no fraudsters working for banks -- but they can work in a store or factory or office).
So, hopefully, he goes damn fuck that was a stupid thing to do I wrecked like two years of my life, and he either straightens out, or at least doesn't do more than only repeat the same small scale stupid shit, and not go for the big leagues moving kilos and stocking ammo -- where he would in fact probably spend 5 or 35 years in federal prison.