The emergency services were called to a house in Berlin's Hermsdorf district
Police in Berlin say two people have died and a third is in a coma after taking part in a group therapy session.
It is said 12 people were involved in the psychotherapy class and were taken to hospital with suspected poisoning.
A 50-year-old doctor has been arrested and has been questioned by detectives, a police spokesman added.
The police say the doctor has admitted to administering a mixture of drugs. Some local reports suggest heroin, amphetamines and ecstasy were used.
The participants were taking part in a group psychotherapy session in Berlin's Hermsdorf district on Saturday, when the emergency services were called by one of those attending, reports say.
When police arrived at the house they found the body of a 59-year-old man. A 28-year-old died later in hospital, a BBC correspondent in Berlin says.
A police spokesman, Michael Gassen, said: "At this session the participants were given substances. What kind of substances these are... has to be investigated now."
The doctor who was arrested admitted handing out the drugs in order to expand the patients' consciousness, he claims to offer "help with spiritual crises" and police say his wife also took part in the therapy session, our correspondent adds.
According to local media reports, the group took a mixture of heroin, amphetamines and ecstasy. It is unclear whether the drugs were injected or taken in tablet form, local reports say.
Some reports indicate a number of the participants who had been taken to hospital were discharged on Sunday.