Paradis pour tous (1982) on Netflix

Doctor Valois has invented the "flashage", a cure for depressed people. After having tested it on monkeys, he tries with a first human patient, Alain Durieux. This is great success, everybody's happy except may be Alain's wife, Jeanne, who's worrying about the changes in Alain's personality. Other patients use the treatment with similar successes, and Valois's happy about it. But the monkeys are changing: non-cured ones are made mad by the overstability and stereotyped behaviour of the cured ones. So are the humans. When Valois realizes he can't stop the process, he decides to "flash" himself.