Passionflower supplies variable quantities of vitamin K and can lessen the effect of oral anticoagulant 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.

Passionflower 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.

Passionflower may potentiate the effects of antiepileptic medications, increasing their sedative and cognitive effects.
– 2001 Dec;2(6):524-532 — Herbal Medicines and Epilepsy: The Potential for Benefit and Adverse Effects. — Spinella M.