Side Effects and Warnings

Blessed thistle is generally considered to be safe when used by mouth in recommended doses for short periods of time, with few reported side effects. Direct contact with blessed thistle can cause skin and eye irritation.

Blessed thistle taken in high doses may cause stomach irritation and vomiting. Blessed thistle is traditionally believed to increase stomach acid secretion, and may be inadvisable in patients with stomach ulcers, reflux disease (heartburn), hiatal hernia, or Barrett's esophagus.

Blessed thistle contains tannins. Long-term ingestion of plants containing tannins may cause gastrointestinal upset, liver disease, kidney toxicity, or increased risk of developing esophageal or nasal cancer. The effects in humans of blessed thistle tannins are not known.

Laboratory studies suggest that blessed thistle may increase the risk of bleeding, although effects in humans are not known. Caution is advised in patients with bleeding disorders or taking agents that may increase the risk of bleeding. Dosing adjustments may be necessary.

Many tinctures contain high levels of alcohol and should be avoided when driving or operating heavy machinery.