Green Tea

Safety, side effects & who should avoid it

Brewed sencha is safe for most healthy adults, and caffeine is the main day-to-day watch-out. Concentrated green tea extract supplements are a different matter: NIH's LiverTox rates green tea a well-established cause of clinically apparent liver injury, and some cases have progressed to acute liver failure, urgent transplant or death. A small number of similar cases have also been reported after drinking green tea infusions rather than taking capsules, so the brewed leaf is not entirely without risk. Separately, drinking any beverage very hot is associated with esophageal cancer — let the cup cool.

Who should avoid it

• Anyone with active liver disease, unexplained raised liver enzymes, or a past liver injury linked to green tea extract; on re-exposure, liver enzymes tend to climb again, and faster.
• People taking bortezomib (Velcade) or another boronic-acid proteasome inhibitor. The evidence is preclinical, but the potential harm — losing the effect of cancer treatment — is serious enough to avoid green tea and its extracts during therapy unless your oncologist says otherwise.
• People taking nadolol, atorvastatin or raloxifene, or another narrow-therapeutic-index drug, without asking a pharmacist first.
• People being treated for iron-deficiency anemia, at least at meals and around iron tablets.
• Anyone taking extract capsules on an empty stomach or in a 'fat burner' blend — the pattern most often reported in green tea liver injury.
• Children should not take green tea extract supplements, and brewed sencha is a caffeinated drink, so keep their intake low or skip it.
• Adults with caffeine sensitivity, arrhythmias, poorly controlled anxiety or insomnia.

Possible side effects

• Caffeine effects: jitteriness, racing heart, headache, disrupted sleep, especially after mid-afternoon.
• Nausea, stomach ache or reflux from strong tea on an empty stomach; the tannins are astringent.
• Reduced iron absorption from plant foods and from iron tablets taken with tea.
• Nausea, constipation, abdominal discomfort and increased blood pressure, reported with green tea supplements (NCCIH).
• Rare but serious: liver injury — jaundice, dark urine, right-upper-abdominal pain, unusual fatigue — usually appearing one to six months after starting an extract. Most people recover after stopping, but acute liver failure, urgent liver transplant and death have all been reported. Stop the product and seek medical care promptly. Susceptibility varies between people and appears partly genetic.
• Drinking any hot beverage at about 140°F (60°C) or above, habitually, is associated with a higher risk of esophageal squamous cell carcinoma in large prospective cohorts. This is about temperature, not about tea.

Medication interactions

• Nadolol (Corgard): in a randomized crossover study in 13 healthy volunteers, a single dose of green tea extract cut nadolol exposure by 28% (low dose) to 40% (high dose); EGCG inhibited the OATP1A2 transporter in cell experiments. NCCIH also lists atorvastatin and raloxifene as drugs green tea can interfere with.
• Bortezomib (Velcade): green tea polyphenols reacted directly with the drug and blocked its anticancer effect in cell and mouse experiments. This is preclinical evidence only, but it is a reason for caution during treatment.
• Warfarin (Coumadin): a 1999 case report describes a man whose INR fell from 3.79 to 1.14 while he was drinking half a gallon to a gallon of green tea a day, then recovered to 2.55 after he stopped. The authors attributed it to vitamin K in the tea. That is a single case at an extreme intake — ordinary cups are unlikely to matter — but tell your anticoagulation clinic about any big change in tea habits.
• Oral iron: in 46 Moroccan women, green tea taken with an iron-fortified meal cut iron absorption by more than 85%. A separate case report describes iron-deficiency anemia that did not respond to oral iron until the patient stopped drinking large amounts of tea.
• Other drugs: laboratory work shows green tea extract can inhibit P-glycoprotein and several OATP transporters, though the few clinical studies available suggest it does not inhibit the major cytochrome P450 enzymes. If you take a drug with a narrow margin between effective and toxic, ask a pharmacist.
• Caffeine stacks: count sencha alongside coffee, energy drinks and stimulant medications.

Pregnancy & breastfeeding

Modest amounts of brewed sencha are generally considered compatible with pregnancy, because the caffeine load is low: common obstetric guidance keeps total caffeine under about 200 mg a day, and a cup contributes roughly 25 to 45 mg. NCCIH advises keeping caffeine intake moderate during pregnancy, and notes that caffeine passes into breast milk, where high maternal intake can make infants fussy and disturb their sleep. Two further points: tea taken with meals reduces iron absorption, which matters more in pregnancy, and green tea extract supplements are not recommended in pregnancy or breastfeeding. Evidence is thin — ask your doctor or midwife.

How much is too much

No official upper limit exists for brewed sencha. The Japanese cohorts reporting benefits recorded intakes of five or more cups a day. A practical ceiling for healthy adults is total caffeine under roughly 400 mg daily — around 9 to 16 cups of sencha, before you count coffee. Extract is a different product. A U.S. Pharmacopeia safety review found published liver-injury case reports at EGCG intakes ranging from about 140 mg to roughly 1,000 mg a day, with substantial differences between individuals that may be partly genetic. High doses have caused trouble in trials: a phase II study giving 800 mg of EGCG daily was halted after 5 of 7 participants developed abnormal liver tests, and in a 12-month trial of 843 mg EGCG a day in postmenopausal women, ALT rose sharply in carriers of certain genotypes.