There is a slight chance of phototoxicity due to the furocoumarins in dong quai. Symptoms can include skin rash, irritation, and extreme sensitivity to the sun or sunburn. If you develop these symptoms, stop using dong quai.
When taken with estrogens, dong quai might increase the risk of estrogenic side effects.
Dong quai has a stimulant effect on the gastrointestinal tract. Talk with your doctor before using it if you have a long-term (chronic) intestinal disease. These can include diverticulitis or irritable bowel syndrome.
If you are pregnant or breastfeeding, don't use dong quai.
Dong quai might slow blood clotting. People who take the blood thinner warfarin shouldn't use dong quai. Doing so may increase the risk of bleeding. Stop taking dong quai at least 2 weeks prior to undergoing surgery. Don't use dong quai with other herbs or dietary supplements that may also slow blood clotting.
Don't use dong quai if you are being treated with radiation therapy. Don't use dong quai if you have a hormone-sensitive cancer.