A report in the November 2009 issue of AIDS Research and Human Retroviruses showed that crystal meth makes HIV reproduce faster-—at least in test tube and animal studies.

It's long been shown that people with HIV who repeatedly use crystal methamphetamine experience more brain damage and cognitive problems—and develop AIDS more quickly—than their positive peers who don't mess with meth. It won't be surprising if the speedier replication data hold true in humans, too. Crystal use also raises the risk of acquiring HIV in the first place. So even before final results are in from human studies, we'd say all signs point to avoiding—or quitting—meth. (For help, click here to read “Kicking Crystal to the Curb.”)