Spermicides are products containing chemicals that damage or kill sperm. Spermicides prevent pregnancy by killing the sperm in a man's semen before the sperm can get to a woman's uterus or fallopian tubes, where fertilization generally takes place. Spermicides come in many different forms, including creams, gels, foams, films, suppositories and tablets.

To decide if a spermicide is the best method of birth control for you, find out how you use them and how effective they are at preventing pregnancy and sexually transmitted disease.

If using a cream, foam or gel, you use the applicator provided to squirt the spermicide into your vagina, covering the cervix. You can have sexual intercourse within a few minutes of application. If you're using a tablet, suppository or film, you insert the product into your vagina by hand and wait at least 15 minutes to allow time for the product to dissolve.

Spermicides aren't long-lasting, so don't wait more than an hour before having sexual intercourse. If it's been more than an hour, you'll need to apply more spermicide. Also apply a dose of spermicide every time you have sexual intercourse, even if less than an hour has gone by. Afterwards, avoid washing or rinsing out your vagina for at least six to eight hours.

Each type of spermicide can differ with regard to how soon you can have intercourse and how long they're considered effective. Check the package for proper instruction on how to use the product you've chosen.

You can also use a spermicide in combination with a barrier form of birth control, such as a diaphragm, cervical cap or condom. Many condoms actually come packaged with a spermicide. Most doctors recommend that you not use a diaphragm without also using a spermicide.

Yes. The protection offered by the spermicide diminishes quickly, making pregnancy possible very soon after use.

Using a spermicide alone is 74 percent effective at preventing pregnancy. This means that out of 100 women using spermicides alone for one year, 26 will become pregnant. If you use a spermicide with a barrier method of birth control, such as a diaphragm or cervical cap, you'll better protect yourself from unwanted pregnancy. Using a diaphragm or cervical cap with a spermicide may be up to 83 percent effective at preventing pregnancy. Condoms and spermicides can be more than 90 percent effective when used together.

It's possible, with repeated use, that you could experience irritation of your vagina and surrounding skin, such as itching, burning or redness. You could develop an allergy to the chemical used in the spermicide. You might also be at greater risk of a urinary tract infection because the spermicide alters the natural balance between good and bad bacteria in your vagina.

No. A spermicide isn't recommended as protection against sexually transmitted diseases (STDs), such as HIV/AIDS, genital herpes and gonorrhea.

Spermicides are inexpensive, ranging from about 50 cents to $1.50 an application, depending on what kind of spermicide you use.

You can buy a spermicide without a prescription. They're available at most drugstores and some supermarkets, usually in the same place where you'd find condoms or feminine hygiene products.

As with any form of birth control, using a spermicide has its advantages and disadvantages:

The relatively high failure rate of spermicides used alone, compared with other forms of birth control, poses a serious risk of unwanted pregnancy. However, spermicides can be effective in preventing pregnancy when used in combination with other forms of birth control. The key to the effectiveness of a spermicide is to use it properly. Apply it every time you're planning to have sexual intercourse.