Coltsfoot has been identified as an unsafe herb.

– Am J Health Syst Pharm 1999 Jan 15;56(2):125-38; quiz 139-41 — Unsafe and potentially safe herbal therapies. — Klepser TB, Klepser ME.

Coltsfoot produces a stimulating effect on the growth of Candida albicans.

– Zh Mikrobiol Epidemiol Immunobiol 2001 Sep-Oct;(5):71-2 — Effect of medicinal plant extracts on the growth of microorganisms — Baronets NG, Adlova GP, Mel’nikova VA.

Chemical studies on the dried, young flowers used in this experiment suggested that the carcinogenicity of coltsfoot is most probably due to senkirkine, a hepatotoxic pyrrolizidine alkaloid.
– Gann 1976 Feb;67(1):125-9 — Carcinogenic activity of coltsfoot, Tussilago farfara l. — Hirono I, Mori H, Culvenor CC.

Do not use if you are pregnant or nursing.
– Hepatic veno-occlusive disease in newborn infant of a woman drinking herbal tea. — Roulet M, et al. — J Pediatr. 1988 Mar;112(3):433-6.