The Methuselah Enzyme by Fred Mustard Stewart, 1970
Drama emanates from a youth clinic in Switzerland, where three pairs have come for a mystery treatment to reverse the aging process. The three wealthy and old clients had brought along their young companions, who are unaware of the true dealings at hand. It's not long before they find out and the tension begins. There's deceit, bribes, alliances and that.
But eventually it becomes clear that the body is in fact not able to age in reverse and the troubling developments that come during their stay are often ghastly.
4/5