Traditionally, blessed thistle is believed to stimulate stomach acid secretion and it may reduce the effectiveness of drugs such as cimetidine (Tagamet®), famotidine (Pepcid®), nizatidine (Axid®), or ranitidine (Zantac®).
Based on laboratory studies, blessed thistle may increase the risk of bleeding when taken with drugs that also increase the risk of bleeding (although effects in humans are not known). Some examples include aspirin, anticoagulants ("blood thinners") such as warfarin (Coumadin®) or heparin, anti-platelet drugs such as clopidogrel (Plavix®), and non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (Motrin®, Advil®) or naproxen (Naprosyn®, Aleve®).
Based on laboratory studies, blessed thistle may increase the risk of bleeding when taken with herbs or supplements that are believed to increase the risk of bleeding (although effects in humans are not known). Multiple cases of bleeding have been reported with the use of Ginkgo biloba, and fewer cases with garlic and saw palmetto. Numerous other agents may theoretically increase the risk of bleeding, although this may not be proven in most cases.
This information is based on a professional level monograph edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com): Ethan Basch, MD (Memorial Sloan-Kettering Cancer Center); Dawn Costa, BA, BS (Natural Standard Research Collaboration); Cynthia Dacey, PharmD (Northeastern University); Paul Hammerness, MD (Massachusetts General Hospital); Sadaf Hashmi MD, MPH (Johns Hopkins School of Hygiene and Public Health); Shaina Tanguay-Colucci, BS (Natural Standard Research Collaboration); Catherine Ulbricht, PharmD (Massachusetts General Hospital); Mamta Vora, PharmD (Northeastern University); Wendy Weissner, BA (Natural Standard Research Collaboration).
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Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use this medication only for the indication prescribed.