There are little to no known side effects or contraindications to drinking green tea for adults. However, the following risks or complications should be made clear:
Caffeine sensitivity - those with severe caffeine sensitivities could experience insomnia, anxiety, irritability, nausea, or upset stomach.
Blood thinners - those taking blood thinners (anticoagulant drugs) such as Coumadin/warfarin should drink green tea with caution due to its vitamin K content. It's also recommended to avoid green tea and aspirin, because they both reduce the clotting effectiveness of platelets.
Other stimulants - if taken with stimulant drugs, green tea could increase blood pressure and heart rate.
Green tea supplements contain high levels of active substances that can trigger side effects and interact with other herbs, supplements, or medications.
Green tea supplements are unregulated by the FDA and may also contain other substances unsafe for health or with unproven health benefits. Always check with a doctor before starting any herb or supplement regimen.
In particular, pregnant or breastfeeding women, those with heart problems or high blood pressure, kidney or liver problems, stomach ulcers, or anxiety disorders should not take green tea supplements or extracts.