Cranberry safety: kidney stones and warfarin interactions
What it found
In people, cranberry was not linked to serious side effects, though stomach upset was common with juice.
The main concerns were kidney stones and possible interactions with the blood thinner warfarin.
What they found
Studies in people
Stomach upset from juice
In people, the most common side effects were stomach and gut complaints, especially with cranberry juice. Some people dropped out of studies because of taste or stomach upset.
Kidney stone risk unclear
Human studies gave conflicting results. Some suggested cranberry might increase risk in people with a history of certain stones, while others suggested it might reduce risk.
Warfarin and moderate intake
In people, moderate amounts of cranberry juice (240 to 480 mL) did not interact with warfarin in clinical studies.
Warfarin and high intake
Some reports suggested that large amounts of cranberry juice (1 to 2 L per day) or cranberry extract (3000 mg per day) could potentially interfere with warfarin. Most case reports involved patients who were seriously ill or taking other medicines.
Animal and lab studies
Animal and lab tests
In rats and mice, cranberry did not cause death at high doses and did not show signs of toxicity in a 14-week study. No reproductive or long-term cancer studies were found.
What the authors conclude
- They concluded that there is no serious risk to human health from cranberry in supplements when used at recommended intake levels.
- They advise that cautionary labeling for warfarin interactions or kidney stone formation is not necessary for cranberry quality standards.
- They note that moderate intakes of cranberry juice have not shown warfarin interactions, but high intakes cannot be overlooked.
How it was done
The U.S. Pharmacopeial Convention wanted to know if cranberry had safety issues that should stop it from being included in quality standards for supplements.
The authors reviewed published clinical studies, animal studies, lab studies, and case reports on cranberry. They looked at side effects, kidney stone formation, and interactions with warfarin.
What it can’t tell you
- The studies in people cannot prove that cranberry causes or prevents kidney stones, only that there is a link.
- Animal and lab results may not apply to people.
- The case reports cannot show cause and effect because most involved other medicines or serious illness.
Who paid
- Conflicts
- The authors declare that they have no conflict of interest.
- Authors work at
- Auburn University, AL
The paper
- Title
- Safety of Cranberry: Evaluation of Evidence of Kidney Stone Formation and Botanical Drug-Interactions
- Type
- ReviewThe authors read earlier studies and describe what they found. It isn’t a new study.
- Evidence
- Studies in people · animals · lab
- Summarised from
- Full text
- Cite
- Madden E, McLachlan C, Oketch-Rabah H, Calderón AI (2021). Safety of Cranberry: Evaluation of Evidence of Kidney Stone Formation and Botanical Drug-Interactions. Planta medica. doi:10.1055/a-1497-6241Free full textPubMed 34015833DOI
Summary written 26 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error