Dong quai is almost never used by itself. It is usually combined with cinnamon and peony. In combination with these herbs, it is used to treat complaints in women that involve excessive estrogen. Estrogen stimulates the growth of endometrial tissue, thickening the womb so it will be ready for the implantation of the egg if it is fertilized. Sometimes small pieces of the endometrium break off and lodge in places where they can interfere with the release of the egg at ovulation or the implantation of the egg after it has been fertilized. Reducing the production of estrogen reduces the growth of these misplaced tissues during the first half of a woman's period. Sometimes this is what is needed for a woman to conceive.
Although the herb vitex is never used to treat infertility in men, dong quai is sometimes to used by men seeking to become fathers. This Chinese herb is a concentrated source of a chemical called ferulic acid. This antioxidant protects sperm from the effects of toxic free radicals, both while the sperm are still in the man's testes and after they have been ejaculated into the womb. Ferulic acid increases the sperm's "swimming ability" and treats the condition of male infertility known as athenospermia, where sperm counts are OK but sperm viability is minimal. Practitioners of Traditional Chinese Medicine usually prescribe dong quai to be taken in the form of a tea.
The active compounds in dong quai are soluble in water, but not in alcohol. That is why a tea can help increase fertility in both women and men, but an alcohol tincture is no good at all. Since dong quai increases menstrual flow, it is important for women to stop taking it once they have become pregnant. Both men and women should avoid dong quai if they have herpes, since it changes the way in which the herpesvirus responds inflammatory triggers.