A scanner darkly

"Bob Arctor is a dealer of the lethally addictive drug Substance D. Fred is the police agent assigned to tail and eventually bust him. To do so, Fred takes on the identity of a drug dealer named Bob Arctor. And since Substance D--which Arctor takes in massive doses--gradually splits the user's brain into two distinct, combative entities, Fred doesn't realize he is narcing on himself."@en
"Bob Arctor is a dealer of the lethally addicitive drug called Substance D. Fred is the police agent assigned to tail and eventually bust him. To do so, he has taken on the identity of a drug dealer named Bob Arctor. And since Substance D--which Arctor takes in mammoth doses--gradually splits the user's brain into two distinct, combative entities, Fred doesn't realize that he is narcing on himself."
"A drug dealer of the future periodically moves away from his spaced-out world to become an informer for narcotics agents until he becomes unable to separate his two personalities."
"A drug dealer of the future periodically moves away from his spaced-out world to become an informer for narcotics agents until he becomes unable to separate his two personalities."@en
"A police officer assigned to capture an elusive drug dealer does not realize that the man he is chasing is really himself--a situation brought about by the drug's capability of producing split personalities in its users."@en