Effectiveness of spermicides

They work by causing the membrane of the sperm cells to break, killing them or slowing their movement. This keeps the sperm from meeting the egg.

Spermicides are one of the least effective family planning methods, with a 29% chance of pregnancy, and as with other methods effectiveness depends on the user. Risk of pregnancy is greatest when spermicides are not used with every act of sex. In general, spermicides may be an appropriate choice for women who need back-up protection against pregnancy (for instance, if they forget to take their birth control pills). Spermicides should not be used alone as the primary method of birth control.