The poisoning occurred during a group therapy session at a doctor's office in the Berlin neighbourhood of Hermsdorf Saturday afternoon.
A 59-year-old man died on the scene and a 28-year-old man died later Saturday evening in hospital after falling into a coma. A third man remained in a coma. The other patients were released Saturday evening.
Police arrested a 50-year-old doctor on the scene.
Media reports say the drug cocktail may have included a mixture of heroin, amphetamines and Ecstasy. There was no word as to whether the deadly combination was injected or offered in tablet form.
A sign in front of the doctor's office offered special psychiatric treatments including psychoactive substances. These reportedly included LSD and psychedelic mushrooms, which are illegal in Germany.
According to the daily Berliner Zeitung, the crime scene was roped off Sunday.