Green tea: Interactions. Contraindications. Pregnancy Use. Practice Points

Adverse Reactions

Due to the caffeine content of the herb, CNS stimulation and diuresis is possible when consumed in large amounts.

One clinical study found an absence of any severe adverse effects when 15 green tea tablets were taken daily (2.25 g green tea extracts, 337.5 mg EGCG and 135 mg caffeine) for 6 months.

Significant Interactions

Controlled studies are not available for green tea, so interactions are speculative and based on evidence of pharmacological activity. Therefore, clinical significance is unknown.

ANTICOAGULANTS

Antagonistic interaction — a case of excessive consumption (2.25-4.5 L of green tea/day) was reported to inhibit warfarin activity and decrease the INR. Intake of large quantities of green tea should be done with caution.

HYPOGLYCAEMIC AGENTS

Caffeine-containing beverages can increase blood sugar levels when used in sufficient quantity (200 mg of caffeine); however, hypoglycaemic activity has been reported for green tea, which could theoretically negate this effect — the outcome of this combination is uncertain, therefore observe patient.

IRON

Tannins found in herbs such as Camellia sinensis can bind to iron and reduce its absorption — separate doses by at least 2 hours. Protein and iron have also been found to interact with tea polyphenols and decrease their antioxidant effects in vitro. The clinical significance of this is as yet unknown.

CNS STIMULANTS

Based on the caffeine content of the herb, high intakes of green tea can theoretically increase the CNS stimulation effects of drugs such as nicotine and beta-adrenergic agonists (e.g. salbutamol); however, the clinical significance of this is unknown — observe patient.

CNS DEPRESSANTS

Based on the caffeine content of the herb, high intakes of green tea can theoretically decrease the CNS depressant effects of drugs such as benzodiazepines; however, the clinical significance of this is unknown — observe patient.

DIURETICS

Based on the caffeine content of the herb, high intakes of green tea can theoretically increase the diuretic effects of drugs such as frusemide; however, the clinical significance of this is unknown — observe patient.

DRUGS METABOLISED BY CYP1A2

The inhibitory effect of caffeine on CYP1A2 may cause other interactions, but this is speculative for green tea.

Contraindications and Precautions

Excessive intake will increase the likelihood of adverse effects due to the caffeine content and therefore is not recommended for people with hypertension, cardiac arrhythmias, severe liver disease, anxiety disorders or insomnia.

Pregnancy Use

Usual dietary intakes appear safe; however, excessive use is not recommended due to the caffeine content of green tea.

Practice Points / Patient Counselling

• Green tea is made from the same plant as black tea, but it contains greater amounts of polyphenols and generally less caffeine.

• Green tea has been found to have significant antioxidant activity and protect against sunburn when applied topically.

• It has antibacterial activity and is used in oral preparations to reduce plaque and improve gingival health.

• Several in vitro and animal studies have shown anticarcinogenic activity for a range of cancers and some epidemiological evidence further suggests cancer protective effects may occur; however, further research is required.

• Epidemiological evidence suggests green tea may reduce cardiovascular disease.

• Preliminary evidence from animal studies has shown that it increases thermogenesis, decreases appetite, reduces inflammation in colitis, reduces glucose levels in diabetes and may be useful in renal failure.

• It is not known whether use will promote weight loss in humans as research results are inconsistent.

Answers to Patients’ Frequently Asked Questions

What will this herb do for me?

Green tea has strong antioxidant effects and some population studies suggest that regular consumption may reduce the risk of cancer and cardiovascular disease. Early research has found it may be useful for sunburn protection, reducing dental plaque formation, colitis, diabetes, renal disease and as an antiseptic. However, further research is required.

When will it start to work?

This will depend on the reason it is being used. Preventative health benefits are likely to take several years of regular daily tea consumption. Effects on oral health care appear to develop more quickly, within 2 weeks.

Are there any safety issues?

Research suggests that green tea is a safe substance when used in usual dietary doses, but excessive consumption may produce side-effects, chiefly because of the caffeine content.