Written by Jianping Liu, Xun Li and the CAM-Cancer Consortium.
Updated February 13, 2014

Green tea (Camellia sinensis)

Is it safe?

No severe adverse effects have been reported in association with medicinal use of green tea.31 According to the current systematic reviews/meta-analysis, drinking green tea appears to be safe at moderate, regular and habitual use. 

Adverse events 

One systematic review identified a total of 34 cases of hepatitis following the consumption of preparations containing green tea between 1999 and 2008, however the review concluded that the toxicity related to concomitant medications could also be involved.32 

According to the reports of clinical trials, adverse events such as diarrhoea,33 nausea and headache 34 are generally considered irrelevant with green tea consumption. However some other researchers suggest that there is presumably a relationship between side effects of nausea, emesis, insomnia, fatigue, diarrhoea, abdominal pain and confusion and green tea use although green tea is well tolerated in most parts.35 

Clinical pharmacokinetic and animal toxicological information indicated that consumption of green tea concentrated extracts on an empty stomach was more likely to lead to adverse effects than consumption after food. 


Green tea can act as an antioxidant and induce superoxide dismutase enzyme, which could scavenge the free oxygen radicals generated by radiotherapy.36 Green tea consumption was shown to increase the plasma concentration of 5-FU in a pharmacokinetic study in rats and in vitro.37 Conclusions from existing pre-clinical studies regarding potential antagonism between bortezomib and green tea components are inconsistent.38 39 


Pregnant women, nursing mothers and patients with cardiac problems are usually advised to avoid or limit their intake to two cups daily.40 People with known allergy or hypersensitivity to caffeine or tannin should avoid green tea.31 


Consumption of high doses of green tea or green tea extract (equivalent to five litres per day) may cause nausea, vomiting, abdominal bloating and pain, dyspepsia, flatulence, and diarrhoea. Excess consumption of caffeine from green tea may also cause central nervous system stimulation such as dizziness, insomnia, tremors, restlessness and confusion, and diuresis, irregularities in heart rate, and psychomotor agitation. 31

Citation Jianping Liu, Xun Li , CAM-Cancer Consortium. Green tea (Camellia sinensis) [online document]. http://ws.cam-cancer.org/The-Summaries/Herbal-products/Green-tea-Camellia-sinensis. February 13, 2014.


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