Most herbs and supplements have not been thoroughly tested for interactions with other herbs, supplements, drugs, or foods. The interactions listed below are based on reports in scientific publications, laboratory experiments, or traditional use. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy.

Interactions with Drugs

Studies of the interactions of black tea with drugs are limited. However, black 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. This combination is commonly used in over-the-counter weight loss products, and may also be associated with other adverse effects, including abnormal heart rhythms, insomnia, anxiety, headache, irritability, poor concentration, blurred vision, and dizziness. 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-adrenergic 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, although 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®), oral contraceptives (OCPs) 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 (cytochrome P-450 isoenzyme 1A2) may be affected by multiple drugs, although the effects in humans are not clear.

Caffeine may lengthen the effects of carbamazepine (Tegretol®) or increase the effects of clozapine (Clozaril®) and dipyridamole (Persantine®, Aggrenox®). Caffeine may affect serum lithium levels, and abrupt discontinuation 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®).

As a diuretic, caffeine increases urine and sodium losses through the kidney, and may add to the effects of other diuretics such as furosemide (Lasix®).

Interactions with Herbs and Dietary Supplements

Studies of black tea interactions with herbs and supplements are limited. However, black 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. This combination is commonly used in over-the-counter weight loss products, and may also be associated with other adverse effects, including abnormal heart rhythms, insomnia, anxiety, headache, irritability, poor concentration, blurred vision, and dizziness.

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 black tea. A combination product containing caffeine, yerbe mate ( Ilex paraguariensis ), and damania ( Turnera difussa ) has been reported to cause weight loss, slowing of the gastrointestinal tact, 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 such as artichoke, celery, corn silk, couchgrass, dandelion, elder flower, horsetail, juniper berry, kava, shepherd's purse, uva ursi, or yarrow.

Interactions with Food

Consumption of food may alter the acute effects of tea on vascular function and blood pressure.

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