The Leech Woman

When a very old African woman offers an ambitious endocrinologist the secret of eternal youth, he decides to take his estranged and no-longer-young-and-beautiful wife along with him on the safari. But then she finds out the true reason for their sudden reconciliation is so that she can serve as his guinea pig.
Written by Larry Cline

http://www.imdb.com/title/tt0054020/plotsummary