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Black Tea: What It Is and What the Research Shows
Black tea is fully oxidized Camellia sinensis — caffeine, theaflavins, and L-theanine. The evidence on blood pressure, cholesterol, and focus.
Black tea is Camellia sinensis leaf that has been fully oxidized, which gives it its dark color, bolder flavor, and signature compounds: caffeine, the polyphenols theaflavins and thearubigins, and the amino acid L-theanine.
Quick facts
| Plant | Camellia sinensis — same plant as green tea, fully oxidized |
| Caffeine | Roughly 40–70 mg per cup (about half of coffee) |
| Key polyphenols | Theaflavins and thearubigins |
| Calming amino acid | L-theanine |
Heart and metabolic markers
A randomized controlled trial found regular black tea reduced blood-pressure variability, and a theaflavin-enriched tea extract trial reported lower cholesterol measurements versus placebo. These are modest, supportive effects — black tea is not a substitute for prescribed treatment.
L-theanine and caffeine
Black tea contains both caffeine and L-theanine. In one 2008 study (Owen and colleagues, Nutritional Neuroscience), 27 volunteers were faster and more accurate on an attention-switching task after 50 mg of caffeine with 100 mg of L-theanine than after placebo. In another (Nobre and colleagues, 2008), 50 mg of L-theanine raised EEG alpha activity at rest compared with placebo; the authors put a cup of black tea at about 20 mg. These are study doses, not a cup of tea, and we make no claim about how a cup will make you feel.
What to keep in mind
Black tea contains caffeine (mind the total if you are pregnant or sensitive), and its tannins can reduce absorption of plant (non-heme) iron when taken with meals — drink it between meals if you are watching iron.
Frequently asked questions
How much caffeine is in black tea?
Roughly 40 to 70 mg per cup, about half that of a typical coffee, depending on leaf and steep time.
Is black tea good for you?
Trials have studied black tea’s effect on blood-pressure variability and cholesterol and report modest, study-specific results, plus calm-alert focus from caffeine paired with L-theanine. It is supportive, not a treatment.
Does black tea affect iron?
Tea tannins can reduce absorption of plant (non-heme) iron when taken with meals. Drink between meals if you are managing low iron.
How was this page put together?
This page cites 4 sources, listed in full below. Published 2026-05-28, last revised 2026-09-28.
This is general information, not medical or dietary advice. We describe what published research reports about an ingredient; that is not the same as what a product will do for you. Evidence on tea and botanicals often comes from small or early-stage studies, sometimes in animals or in a laboratory rather than in people, and results may not match your own. Talk to a clinician before changing anything health-related, particularly if you are pregnant, taking medication or managing a condition. We sell products mentioned on this site, so weigh our pages accordingly and check the sources for yourself.
Sources
- Black tea lowers the rate of blood pressure variation: a randomized controlled trial · American Journal of Clinical Nutrition, 2013
- Cholesterol-lowering effect of a theaflavin-enriched green tea extract: a randomized controlled trial · Archives of Internal Medicine, 2003
- L-theanine, a natural constituent in tea, and its effect on mental state · Asia Pacific Journal of Clinical Nutrition, 2008
- The combined effects of L-theanine and caffeine on cognitive performance and mood · Nutritional Neuroscience, 2008