Flax contains phytoestrogens which have been shown to induce both estrogenic and anti-estrogenic effects but their biological relevance and potency have not been well characterized. Safe and efficacious levels have yet to be established.
– J Pediatr Endocrinol Metab 2002 May;15(5):561-72 — Phytoestrogens: a review of recent findings. — Stark A, Madar Z.

Components in flaxseed with potential hormone-like effects can be transferred to nursing offspring via mother’s milk. Previous investigations demonstrated that maternal feeding of a 10% flaxseed diet during pregnancy and lactation resulted in estrogenic effects on reproductive indices among male and female offspring. Although maternal feeding of flaxseed during lactation appears to be safe with respect to reproductive indices among offspring, future investigation is required to elucidate whether there are any long-term implications with respect to fertility.
– J Toxicol Environ Health A 2001 Dec 7;64(7):567-77 — Exposure to flaxseed or its purified lignan during suckling only or continuously does not alter reproductive indices in male and female offspring. — Ward WE, Chen J, Thompson LU.

Flaxseeds and flaxseed oil can spoil if they are not kept refrigerated. Some possible side effects include diarrhea, gas, and nausea. Flaxseed oil should not be used with other laxatives or stool softeners. People who have inflammatory disease of the intestine, esophagus, or stomach should avoid flaxseed. The immature pods of flaxseed are poisonous.
– American Cancer Society

May cause an internal or external allergic reaction.

Anaphylaxis caused by linseed (flaxseed) intake. — Alonso L, et al. — J Allergy Clin Immunol. 1996 Aug;98(2):469-70.