If you are terminating the individual because he violated company policy related to your drug free workplace policy, then you should be safe to terminate him at any time. Make sure you have such a written policy and that you are consistent in enforcing it (e.g. termination if testing positive).
If you are terminating the individual because his assignment ended, be clear in your documentation that the assignment did truly end. Do not terminate him and then hire a new person to fill that assignment soon after. You want to avoid any indication that you are terminating the employee in retaliation for filing a workers' compensation claim.
Make sure your documentation is in order. The individual will still be entitled to his workers' compensation benefits, regardless of current employment or not. He may be subject to a 50% decrease in wage compensation while out of work due to testing positive; however medical is always 100% paid.
Good luck
Cynthia Monroe, SPHR