They come as creams, films, foams, gels, or suppositories, which you insert deep inside your vagina.
They work by stopping the sperm from moving, so they can’t swim up through the cervix to fertilise an egg.
Spermicides don’t work very well on their own. They’re usually used to back up another birth control method, like a cap, diaphragm, or condom.
Condoms with spermicide
Spermicide-lubricated condoms contain a chemical called nonoxynol-9. This kills sperm – or at least, it stops them moving.
Some people who use condoms lubricated with spermicide find they cause redness, itching, or irritation. Women are more likely to get side effects from spermicide. It can also make them more likely to get urinary tract infections.
If your only option is to use a lubricated condom or not to use a condom at all, it’s still much safer to use the condom.
Most condoms are pre-lubricated not with spermicides, but with simple water-based lubricants. Frequent use of spermicides increases risk of HIV acquisition.
Failure rates
The natural family planning failure rate is high in typical use only, but with spermicides, it is high in both typical and perfect use (21% and 16%, respectively). It is hard to improve effectiveness by simply adhering to correct and consistent use (spermicides are truly the least effective method).