Fennel may cause an allergic reaction.
– Clin Exp Allergy 1997 Nov;27(11):1299-306 — Characterization of allergens in Apiaceae spices: anise, fennel, coriander and cumin. — Jensen-Jarolim E, Leitner A, Hirschwehr R, Kraft D, Wuthrich B, Scheiner O, Graf J, Ebner C.

This case of occupational rhinitis and asthma in an atopic individual involves sensitivity to unique allergens in fennel.
– Ann Allergy Asthma Immunol 1997 Jan;78(1):37-40 — Occupational allergic rhinoconjunctivitis and asthma due to fennel seed. — Schwartz HJ, Jones RT, Rojas AR, Squillace DL, Yunginger JW.

Fennel appeared to act mainly as a diuretic and a natriuretic.
– Clin Exp Hypertens 2001 May;23(4):329-43 — Pharmacological evidence of hypotensive activity of Marrubium vulgare and Foeniculum vulgare in spontaneously hypertensive rat. — El Bardai S, Lyoussi B, Wibo M, Morel N.

The results confirm the oestrogenic activity of the seed extract.
– Indian J Physiol Pharmacol 1985 Jan-Mar;29(1):21-6 — Effect of Foeniculum vulgare Mill. seed extract on the genital organs of male and female rats. — Malini T, Vanithakumari G, Megala N, Anusya S, Devi K, Elango V.

Fennel has been reputed to increase milk secretion, promote menstruation.
– J Ethnopharmacol 1980 Dec;2(4):337-44 — Fennel and anise as estrogenic agents. — Albert-Puleo M.