Pauline Nordin is a trainer, model and licensed nutritionist. Earlier this year, she replaced the frozen peas in her freezer with 2,000 cookies.

The shortbread treats are laden with cannabis—the equivalent of about 1,500 joints. Ms. Nordin, 37 years old, says she can’t recover from her punishing workouts without them. She eats two each night before turning in.

“My...