The sooner the better in this usage, if fertile mucus is present you can bet those little sperms will be swimming hard, and as soon as they get inside the cervix's trapdoor they are safe. There is no guarantee that this will be 100% effective, but by reducing the number of sperm that actually make it inside the cervix, greatly reduces chances that the remaining sperm will actually complete the journey up into the fallopian tube, they have two fallopian tubes to choose from and no way of knowing which is the one holding the egg, find and penetrate the egg. You can see why millions of sperm are released with each ejaculation. But remember it only takes one healthy determined sperm with a bit o'luck on his side to do the job.
Tablets of Vitamin C could also be placed inside the vagina just before intercourse, giving them just enough time to dissolve. Hopefully making things acidic enough to kill the sperm upon ejaculation. This can be irritating however, and another source only recommends this in emergency situations. 12. 1 or 2 500 mg tablets (non-chewable) placed inside the vagina immediately following unprotected sex, or condom rupture, twice a day for 3 days. Soothe afterwards with aloe vera and acidophilus If you are interested in home made spermacides you may want to check out what I have collected about this topic.
If you know fertilizing intercourse took place less than 72 hours ago, you might want to consider Emergency Contraception offered by the medical establishment, as it may be more effective in preventing unwanted pregnancy, find out more for yourself.
Vitamin c can be taken internally as an emergency contraceptive during the time between ovulation and when menstruation is expected. Apparently the ascorbic acid interferes with the production of progesterone. Progesterone is needed to prepare the uterus for pregnancy as well as maintain a pregnancy.
Look for the synthetic form of vitamin c, it should contain no bioflavinoids, and should say in the ingredients ascorbic acid. It is easily obtained from the local grocery store or pharmacy.
Rebecca Chalker in her book, A Woman's Book of Choices lists a dosage of 6 - 10 grams over the course of a 24 hour period for 5 to 10 days, not to exceed 10 days. You will have to determine the correct about for your self; consider body weight, and whether you take vitamin c on a regular basis might need to take 10 gram dose rather than a lower one. You have to decide for yourself, so listen closely to your body.
Jeanne Rose writes in her book Herbs & Aromatherapy for the Reproductive System, "Vitamin C in large amounts seems to encourage the release of the uterine lining. 10-15 grams can be taken daily, at intervals throughout the day, from ovulation until menses to ensure menstrual flow."2
There can be side effects from taking mega doses of vitamin C. Most of the more serious ones occur from extended use. With 10 day maximum used here, these are unlikely to occur. The short term side effects you may experience are constipation or diarrhea, headaches, upset stomach, abdominal cramps, hot flashes, and/or rashes. However, I have not had anyone tell me they experienced any of these symptoms. The only one I experienced was constipation.
People who take anti-coagulants, who have sickle-cell anemia, or who have difficulty metabolizing vitamin c should not take mega doses of vitamin c with out the supervision of their physician.