The inhibitory effect of Old Coke, caffeine-free New Coke, New Coke, Diet Coke and Pepsi-Cola on human sperm motility was studied with a trans-membrane migration method. None of them could decrease sperm motility to less than 70% of control within one hour. A previous study which claimed a marked variation of spermicidal potencies among different formulations of Coca-Cola could not be confirmed. Even if cola has a spermicidal effect, its potency is relatively weak as compared with other well-known spermicidal agents.
Testing of various cola formulas on sperm motility using a trans-membrane procedure did not decrease motility to less than 70% control in a 1-hour period. Diet Coca-Cola had the strongest spermicidal effect followed by Classic Coca-Cola, Caffeine-free Coca-Cola and New Coca-Cola. Since there are no known substances in cola that effect cellular membranes, the results of these tests were not unusual. Other tests have been done using higher dilution of cola which could effect sperm motility and give different results for spermicidal potencies. The results show that cola has little if any spermicidal effect. Its use in postcoital douching is ineffective and could cause complications such as infection.

Hong, C.Y., et al. "The Spermicidal Potency of Coca-Cola and Pepsi-Cola."
Human Toxicology. September 1987 (pp. 395-396).