Holy Basil (Tulsi): Benefits, Research and Side Effects

Holy basil, also called tulsi, is Ocimum tenuiflorum, a close relative of the sweet basil in your kitchen that has been part of Ayurvedic practice in India for thousands of years. Today it is sold as capsules, teas and essential oils, usually marketed as an adaptogen for stress. The short answer on the evidence: a handful of small human studies report favorable results on stress scores and blood sugar, but independent reviewers describe the research as small and mostly of low quality, and no high-quality trial shows it works for any health condition. It also has real cautions for pregnancy, thyroid medicine, blood thinners and diabetes drugs. Here is what is known and what is not.

Peptide Labs sells supplements and hemp products, but none of our current labels lists holy basil or tulsi, and this article does not recommend any product. It is general information, not medical advice.

What is holy basil?

Holy basil is a fragrant, slightly peppery perennial that is native to the Indian subcontinent and now grows in Australia, West Africa and parts of the Middle East. Hindus regard it as a sacred plant and often grow it near shrines, which is where the common name comes from. Cookbooks sometimes call it “hot basil” because of its bitter, spicy taste. Supplements are made from the leaves, stems and seeds, and an essential oil is distilled from the leaves and flowers.

The leaves contain many aromatic compounds, including eugenol (also found in cloves), camphor, eucalyptol and beta caryophyllene. Lab research on these compounds explains a lot of the marketing, but test-tube and animal effects often do not carry over to people, which is the gap this article keeps coming back to.

Holy basil vs sweet basil

Sweet basil (Ocimum basilicum) is the Italian cooking herb. Holy basil is a different species in the same genus with a sharper, clove-like flavor. Using either as a culinary herb is ordinary food use; the cautions below apply to concentrated extracts and supplements.

Holy basil benefits: what the human research shows

The most common holy basil benefits claimed on labels and websites are lower stress, calmer mood, better sleep, steadier blood sugar, lower cholesterol and immune support. Here is how the evidence lines up, with the caveat that most studies are small, short and use different preparations.

Stress, mood and sleep scores

The most cited recent trial, published in 2022, randomly assigned about 100 stressed adults to a branded holy basil extract or a placebo for 8 weeks. The extract group reported lower stress scores and better self-rated sleep quality. The study was funded by a natural-products company, the lead author ran a research company that receives nutraceutical industry funding, and the authors themselves said better objective sleep measurements are needed. That makes it a useful early signal, not proof.

Blood sugar

A small single-blind crossover study in 40 people with type 2 diabetes reported lower fasting and after-meal glucose with holy basil leaves than with placebo, and a 90-day study of 60 patients found slightly larger improvements when holy basil was added to a diabetes drug. These studies are old, small and not designed to change care. The practical point is the reverse of a benefit: because holy basil may lower blood sugar, it could add to the effect of insulin or sulfonylureas and raise the risk of low blood sugar. For other plants marketed for glucose, see our reviews of berberine and cinnamon.

What the systematic review found

A 2017 systematic review by Jamshidi and Cohen gathered 24 human studies on metabolic, cardiovascular, immune and cognitive outcomes. Every study reported favorable results and none reported significant side effects. That sounds strong, but a review is only as good as its studies, and most were small, short and not well controlled, and many were published in journals that are hard to check. The authors called for more work on how holy basil acts, which form to use and who might benefit. The Merck Manual’s consumer page puts it bluntly: the claimed health effects have not been confirmed in high-quality studies in people.

Holy basil side effects and safety

Holy basil appears to be safe for most adults when taken by mouth for up to 8 weeks. The safety of longer use has not been studied. Reported holy basil side effects include nausea and diarrhea. The more important issues are the ones that only show up in specific groups:

• Pregnancy and fertility. In animal studies, large amounts reduced the chance that a fertilized egg implanted and that a pregnancy reached full term. Whether this happens in people is unknown, so anyone pregnant or trying to conceive is advised to avoid it. Safety while breastfeeding has not been studied.
• Thyroid. Holy basil might lower levels of the thyroid hormone thyroxine and could make thyroid hormone medicine less effective.
• Bleeding. It appears to reduce platelet clumping and lengthen clotting time, which may raise bleeding risk during and after surgery.
• Blood sugar. It may lower glucose, which matters for anyone on insulin or sulfonylureas such as glimepiride.

Drug interactions to raise with a pharmacist

• Blood thinners and antiplatelet medicines such as warfarin, heparin, dalteparin, clopidogrel and aspirin.
• Thyroid hormone medicines.
• Insulin and other glucose-lowering drugs.
• Sedatives: animal studies suggest holy basil may increase the effect of some sedatives, including barbiturates.

If you are having surgery, tell your surgical team about holy basil and any other herbs you use, and ask when to stop them.

How holy basil compares with other adaptogens

Holy basil is often sold next to, or blended with, other herbs marketed for stress. The evidence base and safety profile differ for each, so read them one by one: our guides cover ashwagandha, rhodiola and ginseng. If stress and weight are what brought you here, our explainer on cortisol belly separates the hormone science from the marketing. For a calming herb with a different research base, see our passion flower guide.

Choosing a holy basil or tulsi product

Tulsi tea and holy basil capsules are not the same thing. Teas deliver a modest amount of leaf, while extracts are concentrated and sometimes standardized to marker compounds. When comparing labels:

• Look for the species name Ocimum tenuiflorum (also listed as Ocimum sanctum) and the plant part used.
• Be cautious with proprietary blends that hide how much holy basil is in each serving.
• Prefer products with third-party testing; our guide to third-party tested supplements explains the seals.
• Do not swallow holy basil essential oil; it is concentrated and meant for aromatic use.
• Check for other stimulating or sedating ingredients in blends, such as caffeine or L-theanine.

Frequently Asked Questions

Questions about a label? Contact us or read our Buyer’s Guide. Follow the label and ask a healthcare provider before starting any supplement, especially if you take medicines. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Last reviewed October 2026.

Related guides: Ashwagandha Benefits, Side Effects and Safety: What Studies Show, Rhodiola Rosea: Benefits, Side Effects and How to Read the Label, Lemon Balm: Benefits, Stress and Sleep Research, Tea and Side Effects.