The Leech Woman

After traveling to Africa, a woman learns the secret of eternal youth - unfortunately, it entails regularly sacrificing young men.

Synopsis (From the MST3K wiki)
Dr. Paul Talbot is trying to find the secret of eternal youth. A 152 year-old African woman, Mala, approaches Paul and offers him her tribe's secret to long life, a special orchid pollen, but he is doubtful until she demonstrates how she has kept herself alive. He tells the secret to his estranged, alcoholic, and aged wife, June, and gets an guide to help them go on an expedition to Mala's village.

During the dangerous journey, June realizes Paul only wants her along to test the potential potion and becomes enraged. June, Paul, and the guide are captured by Mala's tribe and arrive just in time to see Mala undergo a special ceremony, extracting the pineal hormones of a man with a spiked ring, killing him in the process. She then mixes it with the pollen and drinks it to make her young again. Mala makes an offer for June to use the ring; Paul wants to use the opportunity to escape and "get help". However, June turns the tables and chooses Paul as the man she will kill. She undergoes the ceremony, and she and the guide then create a distraction, steal the ring and pollen, and escape. However, as Mala implied, the effect is temporary; June realizes she must keep killing men to keep young and alive, and so kills the guide after they become lost in the jungle.

When she arrives back in the states, Sally, Paul's shrewishly-voiced nurse, is dating Neil, June's wimpish but mildly attractive lawyer. June uses her new youth to pretend to be Terry Hart, her own niece; she constantly hits on Neil and makes Sally utterly jealous. She also goes around town, using her older form, to trick drunks and con men for their sweet, sweet pineal juice. Neil eventually falls in love with Terry, and decides to end it with Sally. Sally confronts Terry and tries to send her to New York via gun; the two fight, and Terry kills her with the ring.

During a romantic evening she has planned with the now free Neil, a detective investigating one of the men she's killed comes to her home and confronts her; he discovers Sally's body in the closet. Crazed, June rants about the formula for youth and tries to prove it using Sally's pineal juice, however June does not stop aging - in fact, she begins to age even more rapidly. Realizing she's lost everything, she throws herself off of her second-story bedroom balcony and reveals her horrible secret in death.

For the Mystery Science Theater 3000 episode see here 

Tropes used in The Leech Woman:

 * All Take and No Give / Masochism Tango: June and Talbot, and HOW.
 * Aw Look They Really Do Love Each Other: Subverted. Talbot acts the part, but he's just using June straight 'til the end.
 * Blondes Are Evil
 * Calling Card: June accidentally leaves one behind after she murders the sleazy guy. Oops.
 * Darkest Africa
 * Despair Event Horizon: June, after drinking Sally's pineal juice and having her aging accelerate rather than reverse.
 * Dreaming of Things to Come: Just prior to the movie, Malla had dreams of blood, with June as the central figure.
 * Economy Cast: There's no story-related reason that Neil needs to be dating Paul's nurse. However, by having him do so, the producers were able to avoid having to cast two separate actresses as the nurse and Neil's girlfriend.
 * Fountain of Youth: and of Oldth.
 * Gold Digger: Dr. Talbot's a male version.
 * Great White Hunter: David.
 * Hoist By His Own Petard:
 * Immortality Immorality: There's only one place to get pineal juice, and it ain't Wal-mart.
 * I Was Quite a Looker: Oh yeah.
 * Jerkass: Paul Talbot, verging on Complete Monster.
 * Verging?
 * Love Martyr: June is legitimately, whole-heartedly, pathetically in love with Paul.
 * Magical Negro
 * May December Romance: Sans "romance", in Paul and June's case. June and Neil play it a bit straighter - at least in chronological terms.
 * My Niece Myself
 * Non Indicative Name: Though she does, technically, "leech", June is not, alas, a leech-woman. * Sigh*
 * Older Than They Look: Thanks to the naipe, Malla doesn't quite look 152 years old.
 * Out With a Bang: Malla's tribe uses the pineal ceremony to allow old women to go out this way.
 * Phlebotinum Breakdown: Naipe alone = slowed aging, naipe + male pineal juice = youth serum, naipe + female pineal juice = Bride of the Cryptkeeper.
 * Phlebotinum Muncher: ...and a subject suffers Rapid Aging if they don't keep supplementing their doses.
 * Pineal Weirdness
 * Playing Gertrude: June's implied to be approximately in her mid-fifties or so, and much older then Paul. At the time of filming, Coleen Gray was about 37 or 38 - 13 years younger then the man who played her husband. Justified, of course; it's a lot harder to make an older actor look young than to make a younger actor look old.
 * Plot Relevant Age Up: Inverted with Malla, played straight (to ill effect) with June.
 * Quicksand Sucks
 * Stock Footage: "Real Africa...Hollywood Africa!"
 * The Alcoholic: June. "Here's to you, whiskey! Guardian of all frustrated wives!"
 * Although, her alcoholism is forgiveable. Her codependency isn't.
 * Too Dumb to Live: Apparently the restoration of her youth shaved off a few dozen of Malla's IQ points. "Well, I had been having psychic dreams about you becoming a murderer, but since I'm young now, I don't really care any more. Think I'll go get laid - seize the day, you know. Anyway, you've got free rein of the village 'til we execute you and your heavily-armed boyfriend tomorrow! Ta!"
 * Neil Foster probably counts too. A supposedly intelligent lawyer lets himself be instantly seduced away from his fiancee by a client's niece whom he'd never heard of until she appears, and who looks and sounds exactly like a younger version of his client.