Green tea is one of the healthiest beverage in the world. It is made with unfermented tea leaves. That's why it's rich in powerful antioxidants and offers several health benefits.
Although green tea is given Generally Recognized as Safe (GRAS) status by Food and Drug Administration, there are few things you should be aware of before drinking it. Usually, drinking more than 5 cups of green tea per day may be unsafe.
Caution and possible side effects of green tea
May Cause Stomach Irritation
Green tea is generally considered good for stomach. It may prevent gastrointestinal disorders as catechins are absorbed in the gastrointestinal tract. It may also reduce the chance of stomach cancer.
But, drinking strong green tea on an empty stomach may cause stomach irritation for some people. Also, it may cause stomach upset and constipation in some people. 
Green tea contains caffeine
Green tea is a caffeinated drink. Therefore, drinking it in excess amount (more than 5 cups a day), can cause caffeine intolerance and overdose.
According to researchers, a moderate level of caffeine for healthy individual is around 200 mg per day, which if exceeded might cause vomiting, diarrhea, sleep problems, nervousness, headache, etc. [2, 3]
Harmful when consumed during Fasting
When you drink green tea in your fasting period, it might lead you to different kinds of problems. In an experiment done with Beagle dogs it was found that, toxicity was less frequent and less severe in dogs which were fed than with dogs under fasted condition. Although, the experiment has only been performed with Beagle dogs, the effect may be similar with humans. 
May not be good for pregnant women
Drinking excessive green tea during pregnancy is not considered safe. It's because, caffeine can cause contraction of uterine, resulting in miscarriage or premature delivery.
On the other hand, drinking green tea during pregnancy also affect the neural tube development of the fetus. 
If you are pregnant and can't keep yourself away from green tea, then at least try to cut down your daily consumption amount to no more than 2 cups. It is also important for you to consult with your doctor before making any decision even if you are breastfeeding.
Interaction with drugs, herbs and some foods
If you have any illness and you are under medication then it’s better to consult your doctor before you drink green tea.
Green tea may interact with various drugs including amphetamines (stimulant drug), ephedrine (stimulant drug), Nadolol (decrease effectiveness of nadolol), adenosine (used for cardiac stress test), quinolone antibiotics and contraceptive drugs.
You shouldn't take green tea in conjunction with bitter orange, calcium, creatine and herbs that slows blood clotting and harm liver.
Also, milk may reduce some benefits of green tea for the heart and blood vessels ,and it might prevent absorption of antioxidants in tea. 
Side effect of green tea, or is it?
Many early researchers believed that drinking green tea hindered body system of absorbing iron from food, thus resulting in iron deficiency in body. 
However, recent studies have proved that drinking green tea has no association with absorption of iron level in healthy individual, individual with iron deficiency and pregnant women with iron deficiency. [8, 9]