Black tea (Camellia sinensis)

Black tea has not been proven as an effective therapy for any condition, and benefits of specific doses are not established. For heart disease prevention, studies have evaluated 250 to 900 milliliters of tea, consumed daily for up to four weeks. For cognitive performance, an example dose used in research is 400 milliliters of black tea taken three times daily. For dental cavity prevention, 20 milliliters of black tea gargled for 60 seconds daily has been studied.

One cup of tea contains approximately 50 milligrams of caffeine, depending on the strength and size of cup.

People with known allergy/hypersensitivity to caffeine or tannin should avoid black tea. Skin rash and hives have been reported with caffeine ingestion.

Studies of the side effects of black tea specifically are limited. However, black tea is a source of caffeine, for which multiple reactions are reported.

Caffeine is a stimulant of the central nervous system and may cause insomnia in adults, children, and infants (including nursing infants of mothers taking caffeine). Caffeine acts on the kidneys as a diuretic (increasing urine and urine sodium/potassium levels, and potentially decreasing blood sodium/potassium levels), and may worsen incontinence. Caffeine-containing beverages may increase the production of stomach acid, and may worsen ulcer symptoms. Tannin in tea can cause constipation. Caffeine in certain doses can increase heart rate and blood pressure, although people who consume caffeine regularly do not seem to experience these effects in the long-term.

An increase in blood sugar levels may occur after drinking black tea containing high levels of caffeine. Caffeine-containing beverages such as black tea should be used cautiously in patients with diabetes. People with severe liver disease should use caffeine cautiously, as levels of caffeine in the blood may build up and last longer. Skin rashes have been associated with caffeine ingestion. In laboratory and animal studies, caffeine has been found to affect blood clotting, although effects in humans are not known.

Caffeine toxicity/high doses: High doses of caffeine may cause symptoms of anxiety, delirium, agitation, psychosis, or detrussor instability (unstable bladder) may occur. Conception may be delayed in women who consume large amounts of caffeine. Seizure, muscle spasm, life-threatening muscle breakdown (rhabdomyolysis), and life-threatening abnormal heart rhythms have been reported with caffeine overdose. Extremely high doses may be fatal.

Caffeine withdrawal: Chronic use can result in tolerance, psychological dependence, and may be habit forming. Abrupt discontinuation may result in withdrawal symptoms such as headache, irritation, nervousness, anxiety, tremor, or dizziness. In people with psychiatric disorders such as affective disorder or schizoaffective disorder, caffeine withdrawal may worsen symptoms or cause confusion, disorientation, excitement, restlessness, violent behavior, or mania.

Chronic effects: Several population studies initially suggested a possible association between caffeine use and fibrocystic breast disease, although more recent research has not found this connection. Limited research reports a possible relationship between caffeine use and multiple sclerosis, although evidence is not definitive in this area. Animal study reports that tannin fractions from tea plants may increase the risk of cancer, although it is not clear that the tannin present in black tea has significant carcinogenic effects in humans.

Drinking tannin-containing beverages such as tea may contribute to iron deficiency, and in infants, tea has been associated with impaired iron metabolism and microcytic anemia.

Large amounts of black tea should be used cautiously in pregnant women, as caffeine crosses the placenta and has been associated with spontaneous abortion, intrauterine growth retardation, and low birth weight. Heavy caffeine intake during pregnancy may increase the risk of later developing SIDS (sudden infant death syndrome). Very high doses of caffeine have been associated with birth defects, including limb and palate malformations.

Caffeine is readily transferred into breast milk. Caffeine ingestion by infants can lead to sleep disturbances/insomnia. Infants nursing from mothers consuming high levels of caffeine daily have been reported to experience tremors and heart rhythm abnormalities. Components present in breast milk may reduce infants' ability to metabolize caffeine, resulting in higher than expected blood levels. Tea consumption by infants has been associated with anemia, reductions in iron metabolism, and irritability.

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 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®).

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®).

Black 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 (which is the same species as black 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, antidiabetics, antipsychotics, fluconazole, hydrocortisone, levodopa, MAOI antidepressants, phenytoin, proton pump inhibitors (PPIs), riluzole and timolol.

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 green tea. A combination product containing caffeine, yerba 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 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.