Green tea extract supplements: cholesterol, liver injury and interactions

A cup of green tea gives a modest amount of catechins and caffeine. Capsules and powders made from green tea extract can deliver catechins, particularly EGCG, in concentrations far higher than anyone would get from drinking tea. Most of the safety problems described below relate to those concentrated extracts, often taken on an empty stomach and sometimes in weight-loss blends.

The NCCIH summarises the evidence as inconclusive: definite conclusions cannot yet be reached on whether green tea is helpful for most of the purposes for which it is used. In Asian populations, drinking green tea has been linked with lower coronary heart disease risk, but observational studies cannot prove that the tea itself is responsible, because tea drinkers may differ in diet and lifestyle.

In cholesterol studies, green tea modestly reduced total and LDL cholesterol with no effect on HDL cholesterol or triglycerides. That is not a reason to stop a statin, and anyone with high cholesterol should follow the plan agreed with their GP.

Liver injury has occurred in some people using green tea extracts, although it is uncommon. The NCCIH notes that people with a specific variant of a gene that plays an important role in immune function may be more susceptible, and that this variant is found in a minority of the population, so it is hard to predict who is at risk without testing.

Warning signs include yellowing of the skin or whites of the eyes, dark urine, pale stools, persistent nausea, upper-right abdominal pain, itching and unusual tiredness. If any appear after starting a supplement, stop it and seek medical advice the same day. Mention the product to the doctor, and consider Yellow Card reporting.

• People with existing liver disease, or who drink heavily, should ask their doctor before using extracts.

• Avoid combining several supplements at once, so that a problem can be traced.

• Keep a note of brand and dose in case blood tests are needed.

Extract supplements can cause nausea, constipation, abdominal discomfort and increased blood pressure. Because green tea contains caffeine, large intakes can contribute to jitteriness, palpitations and poor sleep, and people with heart rhythm problems or anxiety should be careful.

Green tea may reduce the effectiveness of nadolol, a beta blocker, atorvastatin, a statin, and raloxifene, a bone-protection medicine. More generally, the NHS lists statins, antidepressants, diabetes medicines, blood-pressure medicines, warfarin and hormonal contraceptives as areas where herbal products can interact, and advises checking with a pharmacist before using one.

Never stop a prescribed medicine on the strength of a herbal product. If you take regular medicines, take the supplement bottle to a pharmacist for a quick review.

Moderate caffeine intake is acceptable during pregnancy, and low to moderate consumption while breastfeeding typically poses no problems, although high levels can affect infants. This applies to tea as a drink. Concentrated extracts are a different matter, and the NHS does not recommend herbal medicines during pregnancy or breastfeeding.

The only approved medical use mentioned by the NCCIH is a prescription-type ointment containing green tea extract for genital warts in the US. That is a regulated medicine with defined strength and is not a reason to use extracts elsewhere. In the UK, green tea capsules are typically food supplements, which are not assessed for effectiveness before sale. The MHRA has stated that most herbal products on the market are unlicensed and uneven in quality.