Fertility problems were noticed in cattle ingesting large amounts of red clover. It is believed that the estrogenic properties of the herb were responsible.
– Nord Vet Med. 1984 Mar-Apr;36(3-4):124-9. — Plant oestrogens; the cause of decreased fertility in cows. A case report. — Kallela K, Heinonen K, Saloniemi H.

Red clover is high in isoflavones and may have an effect on hormone levels in the body.
– J Steroid Biochem Mol Biol. 2003 Feb;84(2-3):259-68. — Comparison of hormonal activity (estrogen, androgen and progestin) of standardized plant extracts for large scale use in hormone replacement therapy. — Beck V, Unterrieder E, Krenn L, Kubelka W, Jungbauer A.

Not recommended for use if you are pregnant or nursing.
– Nature. 1958 Oct 25;182(4643):1175. — Reproductive disturbances of Romney ewe lambs grazed on red clover (Trifolium pratense) pastures. — CH’ANG TS.

Use caution if you have any blood diseases as this herb may have an effect on blood sugar and insulin levels.
– Ann Nutr Metab. 2004 Sep-Oct;48(5):343-7. Epub 2004 Oct 20. — Effects of Trifolium alexandrinum extracts on streptozotocin-induced diabetes in male rats. — Amer M, El-Habibi el-S, El-Gendy A.

Red Clover reinforces warfarin action by heterogeneous mechanisms. It should thus not be used in patients on oral anticoagulant and/or antiplatelet therapy.

– Oral anticoagulants and medicinal plants. An emerging interaction — Argento A, Tiraferri E, Marzaloni M. — Ann Ital Med Int. 2000 Apr;15(2):139-43.

Red clover may increase the risk of bleeding or potentiate the effects of warfarin therapy.

– Am J Health Syst Pharm 2000 Jul 1;57(13):1221-7; quiz 1228-30 —
Potential interactions between alternative therapies and warfarin. — Heck AM, DeWitt BA, Lukes AL.

Red clover derived isoflavones were effective in reducing depressive and anxiety symptoms involved with menopause.
– Improvement of postmenopausal depressive and anxiety symptoms after treatment with isoflavones derived from red clover extracts. — Lipovac M, Chedraui P, Gruenhut C, Gocan A, Stammler M, Imhof M. — Maturitas. 2010 Mar;65(3):258-61. Epub 2009 Nov 30.