If you've got no other options
The failure rate of spermicide on its own is pretty high—29% for typical use—but it's better than using nothing at all.
“Boost” another method
You can use spermicide to make another method (like condoms) more effective.
No prescription necessary
If you can't make it to the doctor (or don't want to), you can always use spermicide. It's available at most drug stores and supermarkets. But why not pick up a pack of condoms while you're at it?
Some people are allergic
If you get irritated using spermicide, you're probably allergic to it. Unfortunately, all spermicides and contraceptive gels sold in the United States contain the same active ingredient, Nonoxynol-9. If you’re allergic to that, this method (and condoms with spermicidal lubricant) won’t work for you.
You’re confident your partner is HIV-free.
Women who have a high risk of exposure to HIV shouldn’t use this method. Nonoxynol-9 causes changes in your vadge that can make you more susceptible to HIV. If there’s a chance your guy is sleeping around (or has in the past and has never been tested), use a condom.