Forget Protein Shakes. The Newest Workout Supplement? Marijuana.

Like many other fitness-minded young professionals, a 25-year-old Boston resident named Cameron adheres to a fairly typical preworkout routine. There’s the 20 minutes of stretching, generally followed by some light cardio. And then there’s the component that might be described as less conventional. “I usually smoke a joint,” he says, speaking on the condition that his last name not be published. As marijuana legalization has pushed the drug further toward the mainstream — and a…