The authority Mattilsynet said it had received several reports of adverse events associated with green tea supplements, the majority of which concerned liver damage.
Mattilsynet pointed to high levels of the active substance EGCG (epigallocatechin-3-gallate) in the products, which were far above that obtained from just drinking green tea.
There has been mounting evidence for the various health benefits of green tea, focusing on anti-cancer effects and metabolic, cardiovascular and cognitive benefits.
However there have also been concerns over possible toxicity at high concentrated levels.
Discussing the issue Roy Upton, founder of the American Herbal Pharmacopoeia, told our sister publication NutraIngredients-US: “There are at least 20 cases of liver injury associated with green tea supplements that we know of.”
Dr Luca Bucchini, managing director of Italian firm Hylobates Consulting, said the issue was a "well known problem which has never been really tackled".
The issue was referenced by the European Food Safety Authority (EFSA) in a 2009 guidance document on the safety assessment of botanicals.
"Green tea extracts - within certain amounts - are safe for most people when used as the Norwegian authorities suggest. However, there has been no new risk assessment, and no risk management decision at the EU level. The issue is that we need a regulatory science-based process for botanicals, which we really don't have yet and, unfortunately, this is being held hostage by those who oppose botanicals as such."
Conditions of use
Mattilsynet advised consumers to only purchase products from “serious players” [translation from Norwegian].
It also urged them to read product instructions carefully and avoid consumption on an empty stomach.
Mark Blumenthal, executive director of the American Botanical Council, said: “The presumption is that people are taking these EGCG containing supplements on an empty stomach. Concentrated catechins that hit the liver in a fasting state might have an effect that is different than when the liver is metabolising food. People in a fasting state could be associated with the mechanism of injury, whatever that mechanism is, assuming there is a there.”
Signs of liver damage include dark urine and jaundice.