The players don't need to check the contents of every vile, just what the substance is called. This is also where trust of the person who is giving you the drugs comes in, if they lie and give you something else then you are still guilty and face suspension, but if you were clear on what you agreed to be injected with (which the players weren't), then you could sue the pants of the person who deceived you and possibly get a shortened suspension due to the no significant fault clause.
In my view the players failed on simple due diligence and as such are as guilty as the club, or Dank.