I believe the movie you are thinking of is "Blood Relations".
From an IMDB user comment:
"A woman stays with her fiancÚ at his father's mansion. The father is
a brain surgeon, or at least a brain surgery hobbyist - he has a
surgery room in the basement. Her fiancÚ's ill grandfather also lives
in the house. The father and grandfather seem to be interested in her
at least as much as her fiancÚ.
Lydie Denier is beautiful... She gets drugged several times by drinks,
and doesn't wise up to that. She has odd drug-induced hallucinations
or dreams, or perhaps merely witnesses odd things relating to the
surgeon and his supposedly late wife."
More details from the Internet Movie Database:
From an Amazon commenter:
"Marie (Lydie Denier) is the Eurotrash femme fatale, engaged to Thomas
(Kevin Hicks), a young expectant heir eager to inherit his
grandfather's fortune. Granddad (everyone's favorite Martian, Ray
Walston) is already dying, so all Thomas and Marie need do is kill
Thomas's dad (Jan Rubes) to assure Thomas is the sole heir.
Thomas brings Marie home to meet the family: just dad and granddad.
All three generations of men take a liking to Marie, who resembles
Thomas's dead mother (Rubes's dead wife, Walston's dead daughter).
Things get kinky. Granddad asks Marie to call him "father." Then he
asks her to strip naked and kiss him. So she does."
"Blood Relations" is available for purchase at Amazon:
site:.imdb.com horror drugs "his father" grandfather
I hope this is helpful. If you have any questions regarding my answer,
or believe this is not the movie you are looking for, please don?t
hesitate to ask for further assistance before rating.