For Love Or Murder (1970)

A wealthy, eccentric chemical company owner sends his woman to get an American writer to take a mind control drug, Kemek, to verify its potential. She accomplishes her mission, but falls totally in love with the writer. The chemical magnate, satisfied with the results of the test, rewards her with murder, just as she is about to flee to Naples with her new lover. Her ex-husband and a private detective track the killer behind it all to a remote hideaway to unravel the solution.