Holy Basil

What it is

Holy basil (Ocimum tenuiflorum, also called tulsi or Ocimum sanctum) is a plant used in Ayurvedic medicine and taken as food and tea [1]. Trials test a standardised leaf extract in capsules [2][3].

How it might work

The 2022 stress trial found lower hair cortisol and lower salivary cortisol after a laboratory stress test, which suggests the extract may calm the body's stress hormone response [2]. This is a proposed mechanism and has not been confirmed.

What the research says

The strongest human evidence is for stress. In an 8-week placebo-controlled trial of 100 adults, perceived stress fell 37% on a holy basil extract and 19% on placebo [2]. A 6-week trial of 150 adults with stress symptoms used a different extract and found about 39% more improvement than placebo [3]. A small trial in healthy adults found faster memory and attention test times with leaf extract; its anxiety and cortisol improvements appeared only within the holy basil group [4].

In the 2022 trial, scores on the Athens Insomnia Scale fell 48% on the extract and 27% on placebo. Sleep-tracker data showed no difference between groups [2].

For blood sugar, no human trials were found. Diabetic rats had lower fasting blood sugar after eight weeks of an aqueous extract [5], and a leaf extract lowered blood glucose in diabetic rats over 15 days [6]. Test-tube work found extracts inhibit starch-digesting enzymes [7]. A 2017 review reports human benefits for diabetes but does not name the studies [8].

Doses used in studies

Stress trial: 125 mg twice a day (250 mg daily) of a standardised extract for 8 weeks [2]. Second stress trial: 1,200 mg a day of active compounds for 6 weeks [3]. Cognition trial: 300 mg a day of ethanolic leaf extract for 30 days [4]. Rats: 200 mg/kg for 8 weeks [5] and 500 mg/kg for 15 days [6]. These are study doses, not recommendations.

Safety and interactions

Side effects: in the 2022 trial, adverse effects were similar in both groups. One person on the extract left because of agitation and one on placebo left because of nausea [2]. A 2021 review calls the extracts quite safe, with rare mild nausea reported [1].

Upper limit: no upper intake limit was found in the sources read. The review notes that 1,200 mg a day was tolerated in one trial [1].

Medicines: no human interaction studies were found for blood thinners, antidepressants or immunosuppressants [1]. The stress trials excluded people taking regular medicines [2][3]. Animal studies found blood-sugar lowering, so people taking diabetes medicines should ask a doctor first [5].

Pregnancy and breastfeeding: the review advises against high doses in pregnancy or breastfeeding and says reproductive safety data are inadequate [1].

People with health conditions: the trials excluded people with significant heart, liver, kidney or brain disease, and people with gallbladder, autoimmune or endocrine disease [3][2].

Animal safety: a 90-day rat study found no toxicity at doses up to 1,000 mg/kg a day [1].

Caveats and open questions

The stress trials are small and short. The sleep finding rests on one trial and a self-report questionnaire [2]. Both main trials were run with extracts from the same company [2][3]. No human trial has measured blood sugar. A 2026 blend trial with Rhodiola and Schisandra does not show what holy basil alone does [9]. A completed 30-person registered trial on tulsi, stress and cognition (NCT03184909) has no results in the sources we read.