In an interview with the Telegraph's Seven magazine, to be published next week, Fonda has admitted that she used testosterone to enhance her sex life - something she started experimenting with at the age of 70.
"Here's something I haven't said publicly yet: I discovered testosterone about three years ago, which makes a huge difference if you want to remain sexual and your libido has dropped," she told the Sunday Telegraph.
"Use testosterone. It comes in a gel, a pill or a patch."
She also added that she stopped taking the libido-boosting hormone because "it was giving me acne. It's one thing having plastic surgery, but it is quite another to have adolescence acne. That is going too far."
Fonda's new book, Prime Time, does not actually reveal her use of the hormone, but does devote 50 pages to advice on how to keep your sex life booming as you age.
Fonda may come under fire for promoting testosterone, which remains controversial. In the UK, testosterone treatment is licensed for women who have their ovaries removed, are on oestrogen and have low libido.