If you are a devoted fan who loves to enjoy a cup of green tea anytime, you likely already know about its many benefits. However, it is important to also be aware of the potential adverse effects of green tea consumption, as well as its contraindications, since this beverage is not suitable for everyone.
Much has been written about green tea, and numerous studies have been conducted on it. Green tea is known to be highly beneficial: it contains large amounts of important nutrients, is rich in polyphenols, and is an excellent source of natural antioxidants that prevent cell damage.
What Are the Risks and Side Effects of Excessive Green Tea Consumption?
There are very few known side effects or contraindications associated with green tea consumption in adults. Most side effects can be avoided by consuming moderate amounts of green tea. However, the following risks and complications should be highlighted:
Green Tea Can Cause Stomach Problems
Green tea can irritate the stomach when brewed too strongly or consumed on an empty stomach. It contains tannins, which can increase stomach acid levels. Excess acid may lead to digestive problems such as constipation, acid reflux, and nausea. Brewing green tea with water that is too hot can worsen these effects—water should ideally be around 80°C (176°F), not boiling like for other teas.
Drinking large amounts of green tea may also cause diarrhea. Caffeine has a laxative effect because it stimulates the muscles of the colon to contract more frequently, which can cause stomach cramps. If you suffer from irritable bowel syndrome (IBS), it is best to avoid green tea.
To prevent these side effects, avoid drinking green tea on an empty stomach. Instead, consume it after meals. If you have acid reflux or stomach ulcers, avoid green tea, as it may increase acidity.
Green Tea Can Cause Insomnia
People who are highly sensitive to caffeine may experience insomnia, anxiety, irritability, nausea, or an upset stomach (dyspepsia) if they drink green tea.
Even though green tea contains relatively small amounts of caffeine, it can still cause sleep disturbances in sensitive individuals. Certain compounds in green tea can interfere with the release of hormones like melatonin, which regulates sleep. Green tea also contains L-theanine, a compound that promotes calmness but also increases alertness and concentration—something that can disrupt sleep for some people.
If you are sensitive to caffeine, nutritionists recommend limiting the number of cups of green tea you consume per day. Too much caffeine can disturb your sleep. To avoid these issues, try to reduce caffeinated beverages after lunchtime and stick to a single cup of green tea in the morning.
Green Tea Is Not Recommended for People on Anticoagulants
Those taking blood-thinning medications should consume green tea with caution due to its vitamin K content. Specialists also recommend avoiding green tea in combination with aspirin, as both can reduce the effectiveness of platelets, which are crucial for normal blood clotting.
Additionally, avoid green tea while taking other herbal supplements that may affect blood clotting. These include Angelica sinensis (dong quai), chili peppers, cloves, garlic, ginger, ginkgo biloba, poplar, red clover, turmeric, and willow.
Combining green tea with alcohol can cause adverse reactions, including nervousness, headaches, and rapid heartbeat.
Green Tea May Increase Blood Pressure
When consumed alongside certain medications, green tea may raise blood pressure and heart rate.
Green Tea Can Cause Dizziness and Seizures
High caffeine intake from green tea can cause dizziness because it reduces blood flow to the brain and central nervous system, leading to motion sickness. In rare cases, excessive green tea consumption may trigger seizures or confusion, and in some instances, it can cause tinnitus (ringing in the ears).
Most of these side effects are rare and generally occur only with excessive consumption or in people sensitive to the compounds in green tea.
Green Tea May Affect Iron Levels and Folate Absorption
Like all teas, green tea contains tannins, which can interfere with iron absorption. It is recommended not to drink green tea with iron-rich meals and to wait at least an hour after eating foods high in iron before consuming green tea.
A meta-analysis indicated that this side effect can be particularly dangerous for individuals with anemia or other conditions involving iron deficiency. A case study reported that a 48-year-old businessman developed anemia after consuming 1,500 ml (6 cups) of green tea daily for several years.
To counteract this effect, add lemon to your tea—vitamin C enhances iron absorption. As a precaution, avoid green tea if you have anemia.
As mentioned, tannins in green tea reduce iron and folate absorption. Pregnant women or those trying to conceive may want to avoid green tea.
Green Tea May Increase the Risk of Bone Diseases
Excessive green tea consumption may increase the risk of bone disorders, such as osteoporosis, in susceptible individuals. Compounds in green tea can inhibit calcium absorption, which may harm bone health. Limit intake to 2–3 cups per day if you are prone to bone diseases.
Green Tea: Contraindications
People with heart problems, high blood pressure, kidney or liver issues, stomach ulcers, or anxiety disorders should avoid green tea supplements or extracts.
Green tea may be safe during pregnancy and breastfeeding in moderation—up to 6 cups per day (approximately 300 mg of caffeine). Exceeding this amount during pregnancy may increase the risk of adverse effects and congenital malformations associated with folate deficiency. Caffeine also passes into breast milk, so caution is advised while nursing.
Liver issues have been reported in a small number of individuals taking concentrated green tea extracts. Research suggests that high doses of epigallocatechin gallate (EGCG) can be hepatotoxic. Green tea compounds can also interact with herbal medicines or other medications used to manage health conditions. Therefore, green tea supplements should only be taken under medical supervision.
Green tea has been shown to reduce serum levels and efficacy of certain medications, such as beta-blockers used for hypertension and heart problems. It may also interact with other drugs, so consult your doctor if you are on medication before drinking green tea.
Don’t Consume Green Tea in Excess
There are no major disadvantages to drinking green tea in moderation (1–2 cups per day). However, excessive consumption can harm the kidneys. Drink green tea if you enjoy it—but in moderation.
Remember, green tea and its extracts contain significant amounts of caffeine. Excessive caffeine can lead to tremors, disrupted sleep, and headaches.
