Shock Treatment/Traitement De Choc (1973)

When the executive of the fashion industry Hélène Masson is dumped by her lover, she has a midlife crisis and her friend Gérôme Savignat advises her to spend vacation in the isolated rejuvenation clinic owned by Dr. Devilers and his partner Dr. Berbard. Hélène is welcomed by the happy clients and befriends the Portuguese employee João, who is an illegal immigrant, practicing her knowledge in Portuguese language with him. After the first injection of a formula based on animal blood, Hélène feels very well, But soon Gérôme can not afford to pay the treatment and commits suicide. Then João disappears and Dr. Devilers does not allow her to check-out the clinic. Hélène goes further in her investigation of the clinic and finds the secret of the rejuvenation formula of Dr. Devilers.

IMDb: https://www.imdb.com/title/tt0070827