Yes many people use weed for pain or other medical conditions like cancer and glaucoma but none of those people are playing a extreme contact sport. You also omitted the fact that medical studies have proven that using weed slows reflexes and affects memory both very much needed for football. Should employers that employee people that use heavy equipment stop testing for drug use that includes weed? Those policies are there for the safety of the employees. It's not being on any high horse if you understand why there are policies against using weed or why there are still laws for being Under the Influence while driving or DUI.
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First: Most players are probably not getting high during the game. Second: The NFL is shooting players up with Toradol, Ketorolac, and Marcaine. All of these drugs can cause headaches, dizziness, drowsiness, etc... All of which seem detrimental to playing a high impact sport. On top of that, these drugs can mask concussion symptoms and result in more damage being done to a muscle, ligament, etc.
Given the fact that I don't smoke, I could care less either way... but weed seems like a pretty insignificant thing for the NFL to worry about.
PS. Let's be honest. Most jobs don't test for weed because it affects the employees work performance. They test for weed because their insurance won't pay a claim if the employee has an accident and is found with it in their system.