Marijuana or weed is a drug and the NFL does conduct tests for marijuana during the game season.
The perception of weed among the American public is changing and is perceived by many as a depressant. This is the reason that the NFL has relaxed its stance on marijuana use.
The latest collective bargaining agreement has raised the threshold for what the NFL considers constitutes a positive test during the NFL season to 150 ng/ml from 35 ng/ml.
Now, the NFL issues fines for the first three failed tests and suspension comes into play after the fourth failed test or if a player refuses to participate in a treatment program.
However, NFL players are free to consume weed in any quantity during the offseason and the league does not test players for weed consumption during this time.