I would recommend reading the following site before continuing with vitamin C as an herbal remedy: Sister Zeus - Vitamin C
When I first became pregnant, I was an 18 year old freshman in college and my boyfriend was a 17 year old senior in high school. I researched what we could do and found vitamin C as an option. I called him and told him about it, and he didn't really take this method seriously. However, if you think it won't work, you're wrong. I have used this method a few times. It is not good idea to rely on it as birth control, but if the accident happens, this is a generally safe alternative.
Sister Zeus's site has recommended doses. Over doing it can result in diarrhea. This should not frighten you, just be sure to drink plenty of water to avoid dehydration. We bought generic vitamin C from places like Walgreens and Wal-Mart. The main ingredient should be ascorbic acid, and should not include rose hips. The first time I did this, I was scared and took about 10 to 15 grams orally before I went to bed every night. This was probably way over doing it. I would have to go to sleep right after I took them because it would upset my stomach. After about 6 days I think, I started my period. I began about a week after I missed my period. I had a confirmed pee stick that said I was pregnant. If you miss your period, I don't even really recommend taking a test; it's not important. Vitamin C works by making the lining of your uterus shed. If you're not pregnant, it can still bring on a late period.
The second time I used this method, we decided to insert one gram vaginally. I took the recommended oral amount 6 to 10 grams, only taking 1 or 2 grams every hour. Then at night, my boyfriend would take a pill and push it up to my cervix. He could gently push it against the opening of my cervix. We would not force it in, but generally the natural suction of my body would pull it partially in, or at least stay up against the cervix. Do not over do the vaginal insertion. It can cause irritation on your cervical wall. This is not permanent or dangerous long term, but very uncomfortable. Trust me, I know.
If you cannot get the pill to stay against the cervix, do not worry. Your cervix stays open within the first 8 or 9 weeks of pregnancy and naturally sucks inward (to lure in unsuspecting sperm). Leaving a gram of vitamin C in the back part of your vagina will work.
This method really can work if you start it as soon as possible after your missed period. Some women recommend using parsley to soften your cervix, but I never did this. Sometimes we would have sex after a few days of doing this, and it would bring on my period. Sex generally can soften your cervix. Be sure to use a condom if you have sex though.