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Green Tea Extract Side Effects & Interactions To Be Aware Of (2019)

Green tea has been around for thousands of years, used originally as a medicinal herb and now consumed widely for multiple scientifically proven health benefits. The main benefits of green tea include strong anti-oxidant and anti-inflammatory properties[1], reduction in blood pressure, weight loss assistance and reduction in blood sugar levels.[2]

Green tea is well tolerated in most people and there are currently very few side effects and medical interactions that are associated with green tea extract. Most common side effects come from the caffeine content and these include stomach irritation, headaches, sleeplessness and irritability. More recently, however, research has shown that green tea may have some side effects including the interference with blood pressure and iron deficiencies. [3]

 Green tea extract interactions

Common Side Effects

1. Green Tea and Stomach Irritation:

According to a 2007 review on green tea toxicity, it was found that green tea may cause stomach irritation such as diarrhoea when brewed too strongly or consumed on an empty stomach[4]  Green tea contains a significant amount of caffeine which is known to have a laxative effect.

The authors did, however, conclude that for most people, it would be a challenge to consume the amount of green tea required to trigger a stomach irritation side effect. They further concluded that people who consume less than 500 mg of green tea extract over the course of the day are unlikely to experience side effects.


2. Green Tea and Sleep Disturbance:

Caffeine is found in green tea and has been shown to interfere with sleep. [5] In a recent 2017 study, the effect of standard green tea compared with green tea with lowered caffeine content was investigated. The study included 20 healthy adults, used standard green tea as the control and included 300ml/ day of tea over a period of seven days.

The study found that sleep quality was higher and that self-reported fatigue was significantly lower in those participants that consumed green tea with a lowered caffeine content. It is recommended to rather consume green tea during the day to avoid caffeine-induced sleep disturbances at night.


3. Green Tea and Iron Deficiency:

Green tea including most other herbal teas (excluding rooibos tea) contain compounds called tannins which have been shown to significantly reduce the absorption of iron [6]. A recent review of the adverse effects of green tea extracts concluded that the tannin content in Green Tea may reduce the absorption of iron. As a result, the recommendation stands that iron supplements should not be ingested together with Green Tea supplements [7].

This conclusion is supported by a larger analysis that looked specifically at the impact of tannin consumption from tea on iron absorption.

The review found mixed results but concluded that this particular side effect can be dangerous for people who suffer from anaemia or another disease where iron deficiency is present[8] and recommended that green tea and green tea extracts be taken between meals, not with meals. In a smaller study done in the UK, it was recommended that green tea should be taken before meals and to wait at least 1 hour after eating before drinking green tea again.

Majority of the research on green tea and iron status has been done using brewed green tea. However, given that green tea extracts tend to have higher concentrations of active compounds, the above recommendations should be applied when taking green tea extract supplements.


4. Green Tea and Liver Toxicity:

An increasing number of case studies have identified the ingestion of large amounts of Green Tea extract as having damaging effects on liver function. [9] The limitation of these studies, however, is that the supplements used in the trials were “multi-component products”, and they were unable to pinpoint the liver side effects to the Green Tea Extract specifically.

In a 2007 safety review on green tea extract, the conclusion was that ‘‘available information points to the possibility of liver damage associated with concentrated green tea extracts, especially when taken under fasting conditions.”

However, they also emphasized that there is a wide usage of green tea as a beverage were no link to liver damage has been established. Due to the mixed research findings, the recommendation is for consumers and healthcare professionals to be cautious when using concentrated green tea extract products die to potential liver damage risks[10]

 green tea extract side effects

Common Drug Interactions:

1. Green Tea and Blood Pressure Medication:

Nadolol is a common medication used in the treatment and management of high blood pressure. In a 2014 study, it was found that Green Tea significantly reduced the exposure to nadolol and significantly suppressed the systolic blood pressure-lowering effect of nadolol. The green tea was found to continue to have this effect 48 h after the nadolol was ingested. [11]


2. Green Tea and Blood Thinning Medication:

Vitamin K is a vitamin that plays an important role in blood clotting and is present in green tea. The amount of vitamin K in different green tea products will vary but it is nonetheless recommended that individuals on blood-thinning medication such as warfarin should take green tea products with caution due to its vitamin K content[12].

3. Green Tea and Cholesterol Medication:

Statins are drugs commonly prescribed in the treatment and management of high cholesterol levels. Cholesterol is a fat that is essential in our bodies but high levels of this fat can significantly increase the risk of having a stroke or a heart attack. In a large review of Green Tea interactions on cardiovascular drugs, it was found that green tea significantly reduced the absorption of rosuvastatin. [13]

There is limited further research on the impact of green tea on statins, however, given the above findings caution should be made if you are currently taking or are prescribed rosuvastatin.


4. Green Tea Extract and Iron Supplements:

As discussed above, it is recommended that green tea extracts should not be taken during meals to prevent interference with iron absorption. The same applies to iron supplements and therefore iron supplements should not be ingested together with green tea components [14]


5. Green Tea and Folic Acid:

According to a 2008 study,  green tea extract was found to have an inhibitory effect on folic acid uptake. The authors compared folic acid absorption given green tea, black tea and water. The subjects were given 250 ml of green tea, black tea or water and after 30 min received a single folic acid tablet with another dose of green tea, black tea or water.

The authors found that the consumption of green tea led to a significant 39.9% reduction in 5mg folic acid when compared with black tea or water. [15] There is limited further research on green tea extract and folic acid absorption. However, due to the importance of folic acid during pregnancy, it is recommended to drink green tea with caution due to its potential interaction.


Conclusion: Important Side Effects & Interactions

In summary, the most common Green Tea extract side effects include stomach irritability, sleeplessness, headaches and irritability. It is important to note that these side effects were mostly caffeine-related and it is recommended that green tea extracts should be taken during the day and those with severe caffeine sensitivities should drink green tea with caution.

A tolerable upper intake level of 300mg EGCG/person/day is proposed for food supplements to avoid side effects. [16]

It is also recommended that if you are currently on medication for high blood pressure, blood thinning, high cholesterol and or iron deficiency, that you do not take your medication with green tea extracts as medication interference can occur. However, it is important to note specifically which medication you are taking as not all medications have shown to interact with green tea extracts.

green tea extract supplement for health

Medical Disclaimer

Not intended to treat, diagnose, or cure any disease or ailment. Please read and fully understand the potential adverse effects before using this product. These statements have not been reviewed by the FDA and are not written by a medical professional. Please consult your doctor before using any supplements, especially if you have any medical conditions.




BSc(Med)(Hons) Nutrition & Dietetics Researched & written by Michaela and verified by the Research Team



Citations and Supporting Literature:

[1] Graham, HN. (1992) Green tea composition, consumption, and polyphenol chemistry. Prev Med, 21(3):334-50.

[2] Semeco, A. (2017). 10 Benefits of Green Tea Extract. Healthline. Retrieved from 

[3] Mahan.K, Raymond.J (2017). Krause's food & the nutrition care process. Retrieved from

[4] Mead M. N. (2007). Temperance in green tea. Environmental health perspectives115(9), A445.

[5] Unno, K., Noda, S., Kawasaki, Y., Yamada, H., Morita, A., Iguchi, K., & Nakamura, Y. (2017). Reduced Stress and Improved Sleep Quality Caused by Green Tea Are Associated with Reduced Caffeine Content. Nutrients, 9(7), 777.

[6] Hurrell RF, Reddy M, Cook JD. (1999) Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 81(4):289-95.

[7] Schönthal, A. H. (2011). Adverse effects of concentrated green tea extracts. Molecular Nutrition & Food Research, 55(6), 874–885.

[8] Delimont.M, Haub.M, Lindshield.B (2017) The Impact of Tannin Consumption on Iron Bioavailability and Status: A Narrative Review. Current developments in Nutrition, (1)(2) 1-12

[9] Schönthal, A. H. (2011). Adverse effects of concentrated green tea extracts. Molecular Nutrition & Food Research, 55(6), 874–885.

[10] Sarma, D. N., Barrett, M. L., Chavez, M. L., Gardiner, P. et al., Safety of green tea extracts: a systematic review by the US Pharmacopeia. Drug Saf. 2008, 31, 469–484.

[11] Misaka, S., Yatabe, J., Muller, F., Takano, K., Kawabe, K., Glaeser, H. et al. Green tea ingestion greatly reduces plasma concentrations of nadolol in healthy subjects. Clin Pharmacol Ther. 2014; 95: 432–438

[12] Ware, M. (2017) Green tea: Health benefits, side effects, and research. Retrieved from

[13] Werba, J. P., Misaka, S., Giroli, M. G., Shimomura, K., Amato, M., Simonelli, N., … Tremoli, E. (2018). Update of green tea interactions with cardiovascular drugs and putative mechanisms. Journal of Food and Drug Analysis, 26(2), S72–S77.

[14] Schönthal, A. H. (2011). Adverse effects of concentrated green tea extracts. Molecular Nutrition & Food Research, 55(6), 874–885.

[15] Colalto, C. (2010). Herbal interactions on absorption of drugs: Mechanisms of action and clinical risk assessment. Pharmacological Research, 62(3), 207–227.

[16] Dekant W, Fujii K, Shibata E, Morita O, Shimotoyodome A. (2017) Safety assessment of green tea based beverages and dried green tea extracts as nutritional supplements. Toxicol Lett. 5;277:104-108