Flaxseed can have gastrointestinal side effects such as nausea, constipation, and abdominal cramps.[1] When taken in high doses without sufficient fluids, flaxseed may cause constipation, and a case report of bowel obstruction has also been noted.[2]
The phytoestrogens in flaxseed have weak estrogenic activity. While this is likely to be safe, it is best not to start taking flaxseed during pregnancy, because the effects of these hormones on gestation are uncertain.[2]
The effect of phytoestrogens on sexual development in boys and on male fertility is currently unclear.[3]