Les Quatre Vérités(1966)

Olivier Andrieu married Nicole, the daughter of Lucie and Alfred Trévière, owners of Trévière Laboratories. Olivier works there as a biochemist. He suspects his wife of infidelity and wants to inject her with a truth serum of his own invention. Nicole agrees, provided the family participates in the experiment. After the injection, (almost) all of them feel an irresistible urge to reveal their true nature in front of as many witnesses as possible.