4 possible green tea side effects 1. Stomach upset and constipation In Japan and China, it is common knowledge that you shouldn’t drink green tea on an empty stomach. The green tea tannins increase stomach acid. So, if you drink green tea before eating, it may give you a stomachache, a nauseous feeling, or cause constipation. It is best to drink green tea after a meal or in-between meals. People with peptic ulcers or acid reflux should be especially careful. You can try adding milk if it continuously causes stomachaches. 2. Sensitivity to caffeine Green tea has less caffeine than coffee or black tea, but it still contains caffeine. So, how much is too much green tea? WebMD says more than five cups. Drinking too much green tea can cause caffeine intolerance and minerals overdose. A moderate amount of caffeine is less than 200 mg a day. Too much caffeine can cause the following side effects. Headache Nervousness Sleep ...