Meadowsweet 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. — School of Pharmacy and Pharmacal Sciences, Purdue University, Indianapolis, IN, USA.

Flowers and seeds of meadowsweet show a high level of anticoagulant activity. The seed extracts exert equally high anticoagulating effects when administered in vivo and in vitro.
– Izv Akad Nauk Ser Biol 1993 Jul-Aug;(4):625-8 — A comparative study of the action on the hemostatic system of extracts from the flowers and seeds of the meadowsweet (Filipendula ulmaria (L.) Maxim.) — Liapina LA, Koval’chuk GA.

Do not exceed recommended dosage and use caution if you have an allergy to aspirin, based on the salicylate content. Excessive consumption may cause constipation.
– S. Talalaj & Dr A.S. Czechowicz, Herbal Remedies Harmful and Beneficial Effects, Hill of Content, Melbourne, 1989;

Incidences of hepatotoxicity and nephrotoxicity may be augmented by acetaminophen when concomitantly used with the potentially hepatotoxic herbs Echinacea and kava, and with herbs containing salicylate (willow, meadowsweet), respectively.
– J Clin Pharm Ther 2002 Dec;27(6):391-401 — Herbal medication: potential for adverse interactions with analgesic drugs. — Abebe W.