How effective is spermicide at preventing pregnancy?

“There are two types of spermicide, prescription and over-the-counter options. How effective it is depends on the type you use. If over-the-counter spermicide is used alone, it is only somewhat effective—around 84% when used perfectly and 79% with typical use, that means how real people use it in real life.

In other words:

For people who use spermicide perfectly every single time for a whole year, about 16 in 100 will get pregnant when they don’t want to during the first year of using spermicide.[RW3]

For people who use spermicide as people typically use it, about 22 in 100 will get pregnant when they don’t want to during a whole year of using spermicide.

Prescription spermicide is more effective, 93% effective with perfect use and 86% effective with typical use.”