Neem has a proven ability to prevent pregrancy. Neem oil has also been shown to work well both before and after sex while some purified extracts only worked before sex as preventative. Neem oil appears to be the most effective form of neem for birth control, particularly hexane extracted neem oil. After a single injection of a minute amount of neem oil in the uterine horns, a strong cell-mediated immune response reaction produced a long term (up to 12 months) and reversible block in fertility. There were no changes in menstrual cycles or ovarian function.
Neem oil has also been fount to prevent implantation and may even have an abortifacient effect similar to pennyroyal, juniper berries, wild ginger, myrrh and angelica. The effects were seen as many as ten days after fertilization in rats though it was most effective at no more than three days. In a study on rats, neem oil was given orally eight to ten days after implantation of the fetus on the uterine wall. In all cases, by day 15, the embryos were all completely resorbed by the body. The animals regained fertility on the next cycle showing no physical problems. Detailed study of the rats revealed increased levels of gamma interferon in the uterus. The neem oil enhanced the local immune response in the uterus. Post coital use of neem oil as birth control does not appear to work by hormonal changes but produces changes in the organs that make pregnancy no longer viable.