Read on to learn the 15 side effects of green tea, the right dosage, and precautions you must take.
Green Tea Side Effects
Drinking too much green tea has side effects. Here is a list of the 15 adverse effects you want to be wary about.
1. EGCG In Green Tea Binds Iron, Causing Anemia And Iron Deficiency
Anemia affects 1.62 billion people worldwide (3). It is caused due to iron deficiency and low levels of hemoglobin in the blood (4), (5). Research studies point out that drinking too much green tea daily may cause iron deficiency, leading to anemia and/or low RBC count. Dr. Matam Vijay-Kumar, a Penn State assistant professor, explains that a particular green tea catechin, EGCG (epigallocatechin gallate), binds with iron. This reduces EGCG efficiency as a potent antioxidant and also blocks iron absorption (6), (7). Dr. Matam also advises people with IBD (inflammatory bowel disease)not to consume green tea when taking iron supplements. This is because iron-bound green tea EGCG loses the capacity to inhibit myeloperoxidase (an inflammation-inducing enzyme). This may lead to inflammation and stomach pain, further aggravating IBD. Bottom Line – Green tea EGCG may cause iron deficiency and anemia by blocking iron absorption.
2. Caffeine In Green Tea Interferes With Certain Medications
Caffeine is a CNS (Central Nervous System) stimulator. That is why a cup of green tea makes you feel rejuvenated and re-energized. The catch is, consuming large amounts of green tea while on certain medications may cause side effects. Caffeine is broken down in the body and flushed out. But certain drugs like Cimetidine, antibiotics like Ciprofloxacin, enoxacin (Penetrex), trovafloxacin (Trovan), sparfloxacin (Zagam), norfloxacin (Chibroxin, Noroxin), grepafloxacin (Raxar), Fluconazole, anesthetic drugs like Midazolam, and birth control pills inhibit caffeine breakdown (8). As a result, caffeine continues to be in the body, causing jitters, restlessness, increased heart rate, and, in some cases, arrhythmia (9), (10). Scientists found that green tea caffeine inhibited the metabolism of Clozapine, an antipsychotic drug, leading to clozapine toxicity (11). People on lithium also should avoid consuming green tea. Research shows that vitamin K in green tea inhibits the effects of Warfarin, an anticoagulant (anti-blood clotting) drug (12). Bottom Line – Green tea caffeine may interfere with certain antibiotics, antidepressants, anticoagulants, anesthetic drugs, and birth control pills.
3. Green Tea During Pregnancy May Cause Birth Defects
Several studies show that drinking excess green tea during pregnancy may have a negative impact on the mother and the newborn. Consuming more than 300 mg of caffeine per day increases the risk of hypertension during pregnancy (13). Scientists have also found that caffeine and tannins in green tea can reduce the levels of folic acid. Folic acid, a water-soluble B-vitamin, prevents miscarriages and birth defects like spina bifida (14), (15). Though there is no overall risk associated with tea intake, the anti-folate effects of tea (green) need further study (16). Moreover, drinking excess green tea may increase the risk of preterm birth. Further studies are needed to understand how green tea may cause this effect (17). However, EGCG was found to prevent maternal diabetes-induced neural tube defects (18). It is, therefore, best to talk to your doctor before consuming green tea during pregnancy. Bottom Line – Excessive consumption of green tea during pregnancy may increase the risk of preterm birth, spina bifida, and maternal hypertension. Consult your doctor before consuming green tea during pregnancy.
4. Caffeine In Green Tea May Cause Hypokalemia And Seizures
Hypokalemia is characterized by low potassium levels in the blood. Potassium is important for muscle contraction and the functioning of the body’s proteins. Drinking too much green tea may lower potassium levels, leading to muscle weakness, muscle twitches, muscle cramps, and abnormal heart rhythms (19). Two patients with hypokalemia (low potassium levels) were advised to reduce their green tea consumption. This change alone allowed their potassium levels to return to normal after a few days. However, taking green tea along with other medications was found to induce hypokalemia. Taking green tea alone was not found to cause this effect in a study published in the British Medical Journal (20). Researchers also found that green tea’s potassium-lowering and calcium-lowering properties may have a pre-convulsive (muscle tremor and epilepsy) effect (21). Other studies show that consuming more than 200 mg of green tea per day may increase the risk and frequencies of seizures(22), Though certain studies suggest that caffeine increases the risk of seizures, more research is required. However, caffeine was found to reduce the efficacy of drugs for epilepsy treatment in a study published in Epilepsy & Behavior (23). Some other findings report that people with epilepsy who consumed caffeine in high quantities experienced an increase in seizure frequency(24). Bottom Line – Consuming excessive amounts of green tea may cause hypokalemia and increase the risk and frequency of seizures.
5. Green Tea Extract May Be Toxic For The Liver
Many people consume green tea to lose weight and reap other green tea benefits. But studies show that consuming excessive green tea can backfire and cause liver damage. Epigallocatechin gallate (EGCG) makes up 10% of green tea extract. Higher concentrations of EGCG may cause mitochondrial toxicity, possibly leading to hepatotoxicity (25). A 37-year-old lady with no history of liver damage or alcohol use showed increased inflammation, necrosis (cell or tissue death due to lack of oxygen), and higher aminotransferase levels (a sign of liver damage) after consuming a weight loss pill whose major ingredient was a green tea extract (25). Bottom Line – High concentrations of EGCG found in green tea pills and supplements may cause liver damage.
6. Caffeine Toxicity Causes Headache, Dizziness, And Vomiting
Consuming more than 400 mg of caffeine per day may cause caffeine toxicity. Caffeine toxicity is typically characterized by headache, nausea, vomiting, chills, palpitations, and flushing (26). Higher doses of caffeine may cause blood pressure to drop below normal levels (26). However, people who are sensitive to caffeine may show symptoms of caffeine intoxication at lower doses as well (26). There have also been reports of blood pressure shooting up at normal doses of green tea caffeine (26). Researchers at the Monmouth Medical Center, USA, caution against the use of OTC caffeine pills taken with other nervous system stimulants. These caffeine pills or supplements contain higher than normal doses of caffeine. Taking these may cause atrial fibrillation, and sometimes, even death (26), (27). Note: Caffeine is used to treat migraines. Only a licensed physician has the authority to determine the dose and necessity of caffeine to treat migraines. Bottom Line – Caffeine is toxic in higher doses and may cause headache, nausea, vomiting, low blood pressure, flushing, and atrial fibrillation.
7. Caffeine In Green Tea May Impair Thyroid Function
Green tea catechins are known to reduce the risk of thyroid cancer. But excessive intake of green tea caffeine may impair thyroid function (28). In rat studies, daily caffeine intake of 120-150 mg/kg during pregnancy may cause the baby to lose weight (a case of maternal hyperthyroidism) (29). A study conducted at the National Cancer Center, Tokyo, Japan, showed that green tea consumption might increase the risk of thyroid cancer in postmenopausal women. On the other hand, it may reduce the same risk in postmenopausal women (30). Bottom Line – Green tea catechins in high doses may cause abnormal thyroid function.
8. Green Tea Caffeine May Cause Bone Weakness
The National Osteoporosis Foundation warns that excessive caffeine may prevent calcium absorption (31). Both mice and human studies show that green tea extract (GTE) or higher green tea intake reduces bone accumulation rate, increases fracture risk, and reduces bone mineral density. It may also cause bone loss in the elderly (32), (33), (34), (35). Note: There are several studies that show green tea flavonoids are highly beneficial for the bones. However, in this context, higher doses of green tea may harm bone growth and density. Bottom Line – Excessive consumption of green tea or taking green tea extract pills or supplements may prevent bone growth and increase the risk of fractures.
9. Green Tea Caffeine May Cause Anxiety And Insomnia
While lower doses of green tea may help induce sleep and lower stress, higher doses can lead to insomnia and anxiety (36). The caffeine present in green tea is a nervous system stimulant. Consuming more than 300 mg of caffeine per day may reduce sleep quality and cause insomnia, irritability, depression, anger, and anxiety (37), (38). Bottom Line – Having a lot of green tea per day may increase anxiety and irritation. It may also cause insomnia and reduce the quality of sleep.
10. Green Tea May Prevent Blood Clotting
Warfarin is a common anti-blood-clotting drug administered to treat blood clots and reduce the risk of heart attack and stroke. Scientists have found that green tea caffeine blocks the action of Warfarin. Research shows that caffeine inhibits the metabolism of Warfarin. So, people undergoing Warfarin treatment must avoid caffeine-rich products like green tea extracts or excessive amounts of green tea (39). The vitamin K in green tea also inhibits Warfarin (12). Bottom Line – Avoid consuming green tea if you are on Warfarin or any other anticoagulant drug treatment.
11. Too Much Of Green Tea May Cause Heartburn
Heartburn is a typical symptom of GERD (gastroesophageal disease). GERD is commonly known as acidity and is caused due to smoking, obesity, bad food habits, and excessive green tea consumption. Japanese scientists found that an inactive or sedentary lifestyle, along with an excessive intake of green tea regularly, might increase the risk of GERD (40). Another study conducted on the Asian populations found that a component of tea, theophylline, might cause acid reflux (41). Drinking excessive amounts of green tea is linked to AEE (Asymptomatic erosive esophagitis), a subclass of acid reflux. The incidence of AEE is 3.8 times higher in people who drink tea and also consume alcohol (42). Bottom Line – A sedentary lifestyle and bad food habits, coupled with excessive green tea consumption, may cause heartburn and acidity.
12. Green Tea Caffeine May Cause Stomach Upset And Pain
The International Foundation for Gastrointestinal Disorders reiterates that caffeine possesses laxative properties. Consuming more than 3 cups of green tea per day may cause diarrhea (44). In fact, green tea extract containing high doses of caffeine may cause abdominal pain, jaundice, and dark urine (45). Bottom Line – Consuming more than 3 cups of green tea daily may cause diarrhea, abdominal pain, and dark urine.
13. Green Tea Caffeine May Cause Frequent Urination
No, it’s not the extra water that you consume. It’s the caffeine in green tea. Too much green tea caffeine (more than 4.5 mg per kg) per day causes early urgency of urination and even increases its frequency (46). The diuretic effect of caffeine affects bladder function by increasing neuronal activation (47). Note: Lower doses of green tea may help reduce urinary diseases. Bottom Line – Excessive doses of green tea caffeine may increase urination frequency.
14. Green Tea May Cause Infertility In Men
Green tea polyphenols have therapeutic properties. But, in higher doses, these may cause infertility in men. Scientists found that 10 mg/ml of green tea reduced the reproductive output in Drosophila melanogaster (the common fruit fly)(48). In a study, male albino rats were administered green tea leaf extract for 26 days. After the 26th day, sperm production, sperm motility, and testosterone levels in the rats were reduced (49). Excessive caffeine may damage sperm DNA and negatively affect the male reproductive system (50). Bottom Line – Drinking green tea excessively may damage the male reproductive system, leading to infertility.
15. Drinking Green Tea May Stain Teeth
Drinking green tea after a teeth bleaching treatment causes teeth staining (51). Though there is no research directly linking green tea intake to stained teeth, anecdotal evidence suggests so. Bottom Line – Green tea stains teeth. After a bleaching treatment, it is best to avoid green tea for a few days. These are the 15 green tea side effects backed by science. So, if green tea is dangerous in higher doses, what’s the ideal dose? Find out in the next section.
How Much Green Tea Should You Drink Per Day?
It is best to limit your green tea consumption to 2-3 cups per day. Also, do not exceed the 200 – 300 mg/ml caffeine intake. You may consume decaffeinated green tea but stick to the standard dosage of 2-3 cups of green tea per day. Studies suggest that 338 mg of EGCG per day could be safe for most adults (53). Tip: To know how much green tea caffeine or EGCG you are consuming per cup, check the nutrition label. Some people should avoid green tea. The following section will tell you who.
Who Should Not Drink Green Tea?
Avoid drinking green tea if (54):
you are pregnant. you are on Warfarin treatment (or are taking any other medication). you are experiencing abdominal issues. you have compromised bladder control/high risk of bladder cancer you are planning to sleep within an hour.
Note: Talk to your doctor to know if you should stop consuming green tea altogether or if you can consume it in low doses. Is it okay to drink black tea instead of green tea? Find the answer in the next section.
Green Tea Vs. Black Tea
Green tea and black tea are from the same plant, Camellia sinensis. But green tea is less processed than black tea. This makes all the difference:
Green tea has more antioxidants than black tea. Black tea contains more caffeine.
Both teas have health benefits. But if you are caffeine sensitive, it is best to consume green tea or matcha tea in limited amounts (and totally avoid black tea). In case you are not allowed to drink even matcha tea, we have a list of green tea substitutes.
Green Tea Substitutes
Lemon water Ginger and cinnamon tea Chamomile tea Jasmine tea Spearmint tea Tulsi tea
Does green tea have any side effects for skin or hair? There is no research stating that green tea may adversely affect skin or hair. However, if you experience any such side effects, please stop use and consult your doctor. What is the correct time to drink green tea? While you can enjoy green tea any time of the day, it is recommended to have it early morning or in the afternoon to boost your digestive system and concentration and make the most of your workout.
