I’m looking for the rules covering Magic loss and drug use. Could someone point me in the right direction before Sam patches the Shaman one-to-many times?

I think you need to look to the addiction rules in the BBB, there might be some more info as well in Arsenal's Chemistry section, I can't recall and don't have the books off hand.

Pretty much, you have to go to the highest level of addiction before you start losing essence/magic and then it's still kind of a GM-controlled thing if I recall. High willpower should let you hit most of the addiction tests though and avoid getting that hooked. Maybe.

Checked out those sources, but did not see any mechanics that specifically addressed drug use and decreases in Magic. Unless I'm missing something, then that rating 6 Stimpatch is a good way to keep the Mage standing and slinging.

Stimpatches no longer cause Magic loss in SR4.

There are no such rules in SR4.

Excessive and long-term drug abuse (i.e. a Burnout addiction) can lower Essence, which will cause Magic loss. But it's not as bad as in previous editions.

Unless the group starts abusing it.

They also won't heal drain.

Stim Patches don't heal anything!

They merely allow someone to ignore penalties from Stun damage, in a similar manner to High Pain Tolerance. No mention is made as to an exception for Drain, and High Pain Tolerance also does not make any exception for Drain. As no healing is actually taking place, there is no reason why Stim Patches would not work to alleviate Stun damage penalties caused via Drain.