When you take it as directed, flaxseed doesn't cause side effects. But taking large amounts of it as a laxative without drinking enough fluids can worsen constipation. In rare cases, it can cause an intestinal blockage.
If you are pregnant or breastfeeding, talk with your doctors before taking any herbal medicines. Flaxseed can have mild estrogenic effects. This may harm a pregnancy. But there is no evidence that flaxseed can harm a pregnancy at this time.
You shouldn't take flaxseed if you have ileus or an esophageal or gastrointestinal stricture. You also shouldn't take it if you have an acute inflammatory illness of the intestine, stomach, or stomach entrance.
Flaxseed may delay how quickly you absorb other medicines if you take them at the same time.