The army has updated its policies for discharge upgrades recently when they relate directly to TBI or PTSD. It need to be a direct cause though. If you were self medicating yourself with cannabis to deal with your PTSD, that would be relevant. If you were caught with cocaine or something else hard though, you're going to be shit out of luck.

You want a good lawyer and you want to present your case in person in front of the DRB in DC. Post-service conduct is not of much substance (common myth) although it wouldn't help if you got out and kept getting arrested for drugs. It is vital to start collecting statements from former commanders, first sergeants, and other members of your old unit attesting to your character and performance while you served. Make sure your PTSD diagnosis lines up with the problems that related to your discharge.

Discharge upgrades are rare but, thats because almost every oth discharge submits for an upgrade and 99% of them are just doing it for the very small hope it will work, most people don't have legitimate reasons to have their discharge upgraded.