The belief that Coca-Cola works as an after-sex spermicide is nothing but urban legend, a scientist cautions in the British Medical Journal (BMJ) on Thursday.
Deborah Anderson, a professor in obstetrics and gynaecology at Harvard Medical School, says that Coca-cola douches were sometimes used in 1950s and 1960s America in the belief that the drink's acidity killed sperm.
Soft-drink douches are still used as a post-coital contraceptive ploy in some poor countries, she says.
But, says Anderson, there is no evidence to suggest these unusual forms of contraception work, and plenty of reasons to suggest they could cause harm.
To begin with, Coke is not a very effective spermicide, as it is unlikely to kill the target.
And even if the beverage's secret recipe were lethal, a speedy sperm is likely to outswim the douche and get to cervix first.
In addition, Coca-Cola damages the top layer of cells within the vagina, and makes a woman more prone to sexually transmitted disease. And while it is largely harmless to sperm, soda pop removes healthy bacteria, opening the way to fungal and bacterial infection.
Experiments with other forms of vaginal douches have found an increased risk of pelvic inflammation and ectopic pregnancy, in which a fertilized egg is implanted in the fallopian tube, rather than in the uterus.
Finally, says Anderson, there is the simple fact that there are much more effective and easy-to-use methods of contraception widely available.
Anderson also said there was a reason why she had gone into print.
An old study by her research group, on the impact of Coca-Cola on human semen, had recently been resurrected and had won a spoof Nobel prize, or IgNobel, for offbeat science.
She was afraid that the headlines surrounding this award may have repopularised the legend.