John Stamos -- D.A. to Charge Actor with DUI ... Under the Influence of GHB

John Stamos will be charged with driving under the influence of GHB, the so-called date rape drug, TMZ has learned, but sources close to the actor say he was using the drug as a fitness supplement.

Stamos was arrested in June when cops spotted him driving erratically in Beverly Hills. Authorities say Stamos was so messed up they took him to the hospital. We're told testing revealed there was GHB in his system, along with another non-narcotic drug.

Our Stamos sources say the actor was taking the drug to "lean out body mass" as he prepared for his new TV show. The drug is used by bodybuilders for lean muscle, and for a level of sleep which helps muscles recover.

We're told the criminal case will be filed soon and the charging documents will outline the drugs.

Stamos went to rehab almost immediately after his arrest and completed his 30-day live-in treatment successfully.