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Interactions with Drugs
Studies of the interactions of green tea with drugs are limited. However, green tea is a source of caffeine, for which multiple interactions have been documented.
The combination of caffeine with ephedrine, an ephedra alkaloid, has been implicated in numerous severe or life-threatening cardiovascular events such as very high blood pressure, stroke, or heart attack. Stroke has also been reported after the nasal ingestion of caffeine with amphetamine.
Caffeine may add to the effects and side effects of other stimulants including nicotine, beta-agonists such as albuterol (Ventolin®), or other methylxanthines such as theophylline. Conversely, caffeine can counteract drowsy effects and mental slowness caused by benzodiazepines like lorazepam (Ativan®) or diazepam (Valium®). Phenylpropanolamine and caffeine should not be used together due to reports of numerous potentially serious adverse effects; forms of phenylpropanolamine taken by mouth have been removed from the U.S. market due to reports of bleeding into the head.
When taken with caffeine, a number of drugs may increase caffeine blood levels or the length of time caffeine acts on the body, including disulfiram (Antabuse®), birth control pills or hormone replacement therapy (HRT), ciprofloxacin (Cipro®), norfloxacin, fluvoxamine (Luvox®), cimetidine (Tagamet®), verapamil, and mexiletine. Caffeine levels may be lowered by taking dexamethasone (Decadron®). The metabolism of caffeine by the liver may be affected by multiple drugs, although the effects in humans are not clear.
Caffeine may lengthen the effects of carbamazepine or increase the effects of clozapine (Clozaril®) and dipyridamole. Caffeine may affect serum lithium levels, and abrupt cessation of caffeine use by regular caffeine users taking lithium may result in high levels of lithium or lithium toxicity. Levels of aspirin or phenobarbital may be lowered in the body, although clinical effects in humans are not clear.
Although caffeine by itself does not appear to have pain-relieving properties, it is used in combination with ergotamine tartrate in the treatment of migraine or cluster headaches (for example, Cafergot®). It has been shown to increase the headache-relieving effects of other pain relievers such as acetaminophen and aspirin (for example, Excedrin®). Caffeine may also increase the pain-relieving effects of codeine or ibuprofen (Advil®, Motrin®).
Green tea may contain vitamin K, which when used in large quantities can reduce the blood thinning effects of warfarin (Coumadin®), a phenomenon that has been reported in a human case.
Based on preliminary data, theanine, a specific glutamate derivative in green tea, may reduce the adverse reactions caused to the heart and liver by the prescription cancer drug doxorubicin. Further research is needed to confirm these results.
Based on preliminary data, ingestion of green tea may lower LDL cholesterol and thus may theoretically interact with other cholesterol-lowering drugs.
Other potential interactions may include drugs such as adenosine, alcohol, anticoagulants, antidiabetics, antipsychotics, fluconazole, hydrocortisone, levodopa, MAOI antidepressants, methoxsalen, phenytoin, proton pump inhibitors (PPIs), riluzole, terbinafine, theophylline, and timolol.
Interactions with Herbs and Dietary Supplements
Studies of green tea interactions with herbs and supplements are limited. However, green tea is a source of caffeine, for which multiple interactions have been documented.
Caffeine may add to the effects and side effects of other stimulants. The combination of caffeine with ephedrine, which is present in ephedra (ma huang), has been implicated in numerous severe or life-threatening cardiovascular events such as very high blood pressure, stroke, or heart attack.
Cola nut, guarana (Paullina cupana), and yerba mate (Ilex paraguariensis) are also sources of caffeine, and may add to the effects and side effects of caffeine in green tea. A combination product containing caffeine, yerbe mate (Ilex paraguariensis), and damiana (Turnera difussa) has been reported to cause weight loss, slowing of the gastrointestinal tract, and a feeling of stomach fullness.
As a diuretic, caffeine increases urine and sodium losses through the kidney, and may add to the effects of other diuretic agents.
Based on preliminary data, ingestion of green tea may lower LDL ("bad") cholesterol, and thus may theoretically interact with other cholesterol-lowering herbs and supplements.
Bitter orange, calcium, iron, MAOIs, and tannin-containing herbs and supplements may also interact with green tea.