This 2-inch doughnut-shaped device is made of soft foam and coated with the spermicide nonoxynol 9. It works by covering the cervix and blocking sperm from entering the uterus (it also absorbs the sperm) while the spermicide kills or renders sperm inactive. The sponge is about 85% effective at preventing a first pregnancy, but post-prenancy, that number goes down to 68%.
The Today Sponge is activated by wetting it with water and can be inserted up to 24 hours before having sex and left in place for 6 hours after the act (but not more than 24). In fact, it has enough spermicide in it to work for more than one round.
Since there are no hormones involved, the sponge has few side effects with the rare threat of Toxic Shock Syndrome as the largest. Some women report vaginal irritation, an allergic reaction to nonoxynol 9 or that sex is too messy. It can't be used when you have your period (because it can block your flow) or within the first six weeks after giving birth.