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When taken by mouth: Horsetail is POSSIBLY UNSAFE when taken by mouth, long-term. It contains a chemical called thiaminase, which breaks down the vitamin thiamine. In theory, this effect could lead to thiamine deficiency. Some products are labeled "thiaminase-free," but there is not enough reliable information to know if these products are safe.
When applied to the skin: There isn't enough reliable information to know if horsetail is safe or what the side effects might be.
Special Precautions & Warnings:
Pregnancy and breast-feeding: There isn't enough reliable information to know if horsetail is safe to use when pregnant or breast-feeding. Stay on the safe side and avoid use.
Alcoholism: People who are alcoholics are generally also thiamine deficient. Taking horsetail might make thiamine deficiency worse.
Allergies to carrots and nicotine: Some people with allergy to carrot might also have allergy to horsetail. Horsetail also contains small amounts of nicotine. People with nicotine allergy might have an allergic reaction to horsetail.
Diabetes: Horsetail might lower blood sugar levels in people with diabetes. Watch for signs of low blood sugar (hypoglycemia) and monitor your blood sugar carefully if you have diabetes and use horsetail.
Low potassium levels (hypokalemia): There is some concern that horsetail might flush potassium out of the body, possibly leading to potassium levels that are too low. Until more is known, use horsetail with caution if you are at risk for potassium deficiency.
Low thiamine levels (thiamine deficiency): Taking horsetail might make thiamine deficiency worse.
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