In the 18th century, Casanova was said to have discovered the use of lemons as a contraceptive: a lemon was halved, squeezed, and the rind was placed over the uterine orifice like a cap.
Lemon juice contains a weak concentration of citric acid which immobilises sperm cells or slows them down. As a result, vaginal douches containing lemon juice can influence fertilisation when the fluid comes into contact with the sperm before they make it to the womb. A solution of one to two tablespoons of lemon juice combined with 250 ml of water was used.
Source: Norman E. Himes, Medical History of Contraception, 1936