Cranberry pills are relatively safe but may cause stomach upset in some people. Anyone with a sensitivity or allergy to salicylates, a history of kidney stones or those taking the blood-thinning drug Warfarin may want to avoid cranberry supplements.

Cranberry supplements are considered safe for most people, although pregnant and breastfeeding women should ask their doctor before taking any supplement, including cranberry. Cranberry has relatively high levels of oxalate, chemicals that may raise the risk of kidney stones in some people.

Cranberry products are generally thought to be safe. However, if consumed in very large amounts, they can cause stomach upset and diarrhea, particularly in young children. Little is known about whether it’s safe to use cranberry for health purposes during pregnancy or while breastfeeding.

Common side effects of cranberry include stomach upset, reflux, nausea, diarrhea, stomach pain (rare), headaches, elevation of blood glucose levels, kidney stone formation, and increased risk for urinary tract cancer due to oxalate stones. Avoid cranberry juice and supplements if pregnant or breastfeeding.