Holy basil / tulsi

What it is

Holy basil, or tulsi, is an aromatic plant native to the Indian subcontinent whose leaves are used in Ayurvedic and Siddha medicine and drunk as a tea. Supplements are made from leaf extracts, sometimes standardised and branded. It is a different species from the sweet basil used in cooking.

What the research says

Lead with the state of the evidence: it is weak and probably one-sided. A 2017 systematic review found 24 human studies, and noted that every single one reported a favourable outcome and none reported a significant adverse event, which is itself a signal that the literature may be selectively reported. A professional reference summarising the field says there are few high-quality human studies showing holy basil works for any condition. One registered, industry-funded randomised trial in 100 stressed adults found improvements in perceived stress and insomnia scores over 8 weeks. Against that, holy basil inhibits platelet aggregation, may lower thyroxine, and animal work shows it interferes with implantation and pregnancy.

Evidence grade: Limited evidence.

What goes wrong

Holy basil inhibits platelet aggregation and prolongs clotting times, which may raise bleeding risk around surgery and in people taking antiplatelet or anticoagulant drugs. (Source 1)

• Expert review, not systematic, Low certainty.
• Size: not applicable.
• Who: adults, particularly surgical patients and people on antiplatelet or anticoagulant medicines.
• How long: reference updated July 2025.
• Result: No rate given; described as inhibition of platelet aggregation and prolonged clotting times, with increased bleeding risk during and after surgery.
• Funding: not stated.

Holy basil appears to inhibit platelet aggregation and prolong clotting times, which may increase the risk of bleeding during and after surgery.

In animal studies, large doses of holy basil reduced implantation of a fertilised egg and the chance of carrying a pregnancy to term; whether this happens in humans is not known. (Source 1)

• Animal study, Low certainty.
• Size: not stated.
• Who: animals given large doses of holy basil.
• How long: not stated.
• Result: No rate given; reduced implantation and reduced likelihood of a full-term pregnancy at large doses in animals.
• Funding: not stated.

In animal studies, large doses of holy basil reduced the chance that a fertilized egg would become attached to the uterus and that the pregnancy would last for a full term.

Holy basil may lower thyroxine and may reduce the efficacy of thyroid hormone medicines. (Source 1)

• Expert review, not systematic, Low certainty.
• Size: not applicable.
• Who: people with hypothyroidism or taking thyroid hormone replacement.
• How long: reference updated July 2025.
• Result: No effect size given; described as a decrease in thyroxine levels with worsening of hypothyroidism, and reduced efficacy of thyroid hormone medications.
• Funding: not stated.

Holy basil might decrease levels of the thyroid hormone thyroxine, worsening hypothyroidism.

The US Food and Drug Administration warns that unapproved ayurvedic products, the category tulsi is most often sold in, can contain harmful levels of heavy metals and cause poisoning. (Source 2)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: US consumers of ayurvedic products.
• How long: warning page dated December 2025.
• Result: No rate is given. The agency's own wording attributes the harms to chronic exposure and accumulation of heavy metals rather than to a single use, and lists high blood pressure, kidney injury, fatigue, gastrointestinal symptoms such as abdominal pain, constipation, diarrhea, nausea and vomiting, and nervous system-related symptoms.
• Funding: government agency warning.

FDA warns consumers that using unapproved ayurvedic products containing harmful levels of heavy metals may cause heavy metal poisoning.

The FDA states that heavy metals accumulating from ayurvedic products can make adults and children very sick, and that large ingestions over a short period can be life-threatening. (Source 3)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: US consumers of unapproved ayurvedic products, adults and children.
• How long: chronic exposure, and acute ingestions over short periods.
• Result: No rate or dose threshold is given. The agency describes chronic accumulation causing high blood pressure, kidney injury, fatigue, gastrointestinal symptoms and nervous system-related symptoms, and states that large acute ingestions can be life-threatening.
• Funding: government agency warning.

Adults and children can become very sick when heavy metals accumulate in their bodies over time, and large acute ingestions over short periods of time can be life-threatening.

What the evidence supports

A single registered, industry-funded randomised placebo-controlled trial of 100 stressed adults reported greater improvement in perceived stress and insomnia scores and lower hair cortisol on a standardised holy basil extract than on placebo over 8 weeks; one trial is not a literature. (Source 4)

• Randomized trial, Low certainty.
• Size: 100 volunteers.
• Who: adults aged 18 to 65 experiencing stress.
• How long: 8 weeks.
• Result: Perceived Stress Scale p = 0.003; Athens Insomnia Scale p = 0.025; hair cortisol at week 8 p = 0.025.
• Funding: industry-funded: the trial was funded by Natural Remedies Pty Ltd, the supplier of the tested extract, and was run by a contract research organisation that takes nutraceutical-industry funding.

Limit of this finding: This is one trial, not a body of evidence, and the company that sells the tested extract paid for it. The published abstract reports p-values but no effect sizes, so the size of the benefit cannot be judged from it. A reader should also know that a re-check of this entry found the publisher's rendered web page returning p = 0.09 for the primary stress outcome on repeated reads while the publisher's own deposited abstract returns p = 0.003; the deposited figure is recorded here and a human should confirm it against the PDF.

Compared to the placebo, Ocimum tenuiflorum supplementation was associated with greater improvements in PSS (p = 0.003) and AIS (p = 0.025) scores; and at week 8, concentrations in hair cortisol were also lower (p = 0.025).

What the evidence does not support

A professional clinical reference states that there are few high-quality human studies showing holy basil is effective for any health condition. (Source 5)

• Expert review, not systematic, Low certainty.
• Size: not applicable.
• Who: general adult population.
• How long: reference updated July 2025.
• Result: No effect estimate; the reference describes the human trial base as small, with one 100-person randomised trial, one 40-patient cross-over study and one 60-patient 90-day comparison.
• Funding: not stated.

There are few high-quality studies in humans demonstrating that holy basil is effective for treating any health condition.

Where the evidence is mixed

A systematic review of human studies of tulsi found 24 studies across metabolic, cardiovascular, immune and cognitive outcomes, all of which reported favourable results and none of which reported a significant adverse event. (Source 6)

• Systematic review, Very low certainty.
• Size: 24 human studies.
• Who: adults in studies of metabolic disorders, cardiovascular disease, immunity and neurocognition.
• How long: not stated in the abstract.
• Result: All 24 studies reported favourable clinical outcomes; no study reported a significant adverse event. No pooled effect estimate was reported.
• Funding: not stated.

All studies reported favourable clinical outcomes with no studies reporting any significant adverse events.

The holy basil stress trial was funded by the supplement's manufacturer, and two authors were connected to a contract research organisation funded by nutraceutical companies. (Source 7)

• Randomized trial, Low certainty.
• Size: 100 volunteers.
• Who: adults aged 18 to 65 experiencing stress.
• How long: 8 weeks.
• Result: Not an effect size: a declared funding and conflict-of-interest arrangement that bears on how the result should be weighted.
• Funding: industry-funded (Natural Remedies Pty Ltd)

This study received funding from Natural Remedies Pty Ltd.

Where the research disagrees

Whether tulsi is an effective treatment for chronic disease

• Jamshidi and Cohen, systematic review of human tulsi studies, systematic review of 24 human studies, all of which reported favourable outcomes: The reviewed studies reinforce traditional uses and suggest tulsi is an effective treatment for lifestyle‐related chronic diseases including diabetes, metabolic syndrome, and psychological stress. (Source 6)
• Merck Manual Professional Edition, expert clinical reference summarising the same small trial base: There are few high-quality studies in humans demonstrating that holy basil is effective for treating any health condition. (Source 5)

How much

• Reference intake: No reference intake exists. Holy basil is a culinary and medicinal herb, not an essential nutrient, and the 2017 systematic review states that the dosage and dose form still need to be clarified. (Source 6)
• Upper limit: No upper limit or acceptable daily intake has been set by any body reached in this search. A professional reference states only that holy basil appears safe for most people by mouth for up to 8 weeks and that longer use has not been studied. (Source 5)
• Studied: 125 mg of a standardised Ocimum tenuiflorum extract twice daily for 8 weeks in a randomised placebo-controlled trial of 100 stressed adults. (Source 4)
• Studied: A 90-day study in 60 patients compared glibenclamide alone with glibenclamide plus holy basil. (Source 5)

A common belief, and what the research shows

The belief: Tulsi is a gentle herb with no side effects, so there is nothing to watch out for.

What the research shows: That belief partly comes from the literature itself: the systematic review recorded that 'All studies reported favourable clinical outcomes with no studies reporting any significant adverse events.', which says as much about how tulsi has been studied as about the herb. Clinical references do list concerns: 'Holy basil appears to inhibit platelet aggregation and prolong clotting times, which may increase the risk of bleeding during and after surgery.' and 'Holy basil might not be safe when taken by women who are pregnant or trying to become pregnant.'

Questions and answers

What is it?

Tulsi, or holy basil, is a leafy aromatic plant from the Indian subcontinent, revered in Ayurvedic and Siddha medicine. The leaves are drunk as a tea and taken as extracts in capsules. It is a relative of, but not the same as, the sweet basil used in cooking. (Source 6)

What does it do in the body?

Human studies have looked at stress, sleep, blood glucose, blood lipids, immune markers and cognition. A registered trial found lower perceived stress and insomnia scores and lower hair cortisol after 8 weeks, but a clinical reference summarising the field says few high-quality human studies show it works for anything. No mechanism is settled in humans. (Source 5)

Is it good or bad for you?

Context matters. Short-term use in adults appears reasonably well tolerated in the trials done so far, and the reported effects on stress scores were favourable. But it is not neutral: it inhibits platelet aggregation, may lower thyroxine, and animal work at large doses interfered with implantation and pregnancy, so clinicians advise against it for people trying to conceive. (Source 1)

How do you get more of it?

Holy basil is taken as the leaf, as a tea, or as a standardised extract in capsules. The registered stress trial gave adults 125 mg of a standardised Ocimum tenuiflorum extract twice a day for 8 weeks. This is a description of what trials used, not a recommendation. (Source 4)

If it is harmful, what reduces it?

Where holy basil is a problem it is a problem of exposure rather than accumulation, so what the literature describes is avoidance rather than removal. Clinical references advise against its use in people trying to conceive and flag bleeding risk around surgery and with antiplatelet or anticoagulant drugs. No clearance or washout protocol has been studied. (Source 1)

Why might someone be low in it or missing it?

Nobody is deficient in holy basil. It is a herb, not a nutrient the body requires, so there is no low or missing state to explain. The systematic review does not describe a deficiency; it says instead that even the right dose and dose form are still unclear. (Source 6)

We searched: Searched the 2017 systematic review of human tulsi studies and the Merck Manual Professional Edition entry for any description of holy basil deficiency, depletion or insufficiency; none exists.

Which whole foods contain it or feed it?

The whole food is the tulsi leaf itself, eaten fresh or, most commonly, infused as a tea. The human studies reviewed all involved ingestion of tulsi in one form or another, with leaf and leaf extract the usual preparations. There is no separate food that supplies it. (Source 6)

What happens if you do not have it?

Nothing is known to happen. There is no reported consequence of not consuming holy basil, because it is not an essential nutrient and no deficiency syndrome has been described. The clinical question in the literature is only whether adding it changes anything, and there the evidence is thin. (Source 5)

We searched: Searched the 2017 systematic review and the Merck Manual Professional Edition entry for any account of what follows from absence of holy basil in the diet; nothing addresses it.

How can you test for it?

There is no validated test of holy basil status in a person, and none is described in the reviewed literature. The measurable things in trials are outcomes rather than the herb itself, such as hair cortisol and questionnaire scores. Where testing is discussed for ayurvedic products it is product testing for heavy metals, not testing of people. (Source 2)

We searched: Searched the 2017 systematic review of human tulsi studies, the 2022 randomised trial and the Merck Manual Professional Edition entry for a blood, urine or tissue assay of holy basil intake or status; none exists.

References

1. Merck Manual Professional Edition. Holy Basil - Dietary Supplements - Merck Manual Professional Edition (Adverse Effects and Drug Interactions). 2025. Read the source
2. U.S. Food and Drug Administration. FDA warns about heavy metal poisoning associated with certain unapproved ayurvedic drug products. 2025. Read the source
3. U.S. Food and Drug Administration. FDA warns about heavy metal poisoning associated with certain unapproved ayurvedic drug products. 2025. Read the source
4. Frontiers in Nutrition. A randomized, double-blind, placebo-controlled trial investigating the effects of an Ocimum tenuiflorum (Holy Basil) extract on stress, mood, and sleep in adults experiencing stress. 2022. DOI 10.3389/fnut.2022.965130. Read the source
5. Merck Manual Professional Edition. Holy Basil - Dietary Supplements - Merck Manual Professional Edition. 2025. Read the source
6. Evidence-Based Complementary and Alternative Medicine. The Clinical Efficacy and Safety of Tulsi in Humans: A Systematic Review of the Literature. 2017. PMID 28400848, DOI 10.1155/2017/9217567. Read the source
7. Frontiers in Nutrition. A randomized, double-blind, placebo-controlled trial investigating the effects of an Ocimum tenuiflorum (Holy Basil) extract on stress, mood, and sleep in adults experiencing stress - Conflict of interest statement. 2022. DOI 10.3389/fnut.2022.965130. Read the source