The 2017 systematic review of holy basil found 24 human studies, and not one reported a significant adverse event. That is reassuring for short use, and it is silent on the 3 cautions below.
Every caution on this page names where it came from, because most of them came from animals.
Quick Answer: What Are Holy Basil's Side Effects?
In human studies holy basil has been well tolerated; a 2017 review of 24 studies found no significant adverse events. The cautions come from elsewhere: blood sugar may fall further in people on glucose-lowering drugs, rat and rabbit studies found reduced fertility, and 1 mouse study found lower thyroxine. Avoid it in pregnancy.
Key Takeaways
- The 2017 review of 24 human studies found no significant adverse events.
- Rat and rabbit studies found lower sperm counts; 1 rat study saw recovery.
- 23 PubMed records concern fertility or pregnancy, and 0 are human trials.
- 1 mouse study in 1998 found lower thyroxine after 15 days.
- The 1996 human trial lowered fasting glucose 17.6%, which matters with diabetes drugs.
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What the Human Studies Reported
In people, holy basil has a clean short-term record. The trials that reported safety found nothing serious, and the only systematic review of human use reached the same conclusion. The limit is time: none of this evidence runs longer than a few months.
- 2012 stress trial: no adverse events reported over 6 weeks.
- 2022 stress trial: well tolerated, no major adverse effects over 8 weeks.
- 2017 review: 24 studies, none with a significant adverse event.
- What is missing: any trial longer than a few months.
The review searched Cochrane, Embase, Medline and Indian databases for human studies in which people ingested tulsi.[1]The Clinical Efficacy and Safety of Tulsi in Humans: A Systematic Review of the Literature — Evidence-Based Complementary and Alternative Medicine (2017) View source What the herb is and why people take it is in our holy basil overview.
The Fertility Caution, and Where It Comes From
This is the caution most pages mention in 1 line and none explains. It comes from animal studies, some dating to the 1960s and 1970s, in which holy basil was screened as a possible contraceptive. PubMed holds 23 records on it and 0 human trials.
| Animal | What was given | What happened |
|---|---|---|
| Male rats, 2002 | 250 mg/kg of a solvent leaf extract, 48 days | Lower sperm count and motility, normal 2 weeks after stopping |
| Male rats, 2009 | The same extract and dose | No implantation in females mated with treated males |
| Male rabbits, 2010 | 2 g of fresh leaves a day, 30 days | Lower sperm count, altered hormones |
| Female rats, 2019 | 500 mg/kg of leaf extract, 15 days | Raised progesterone, reduced ovulation |
The rat changes were attributed to an anti-androgen effect and reversed within 2 weeks.[2]Reversible anti-fertility effect of benzene extract of Ocimum sanctum leaves on sperm parameters and fructose content in rats — Journal of Basic and Clinical Physiology and Pharmacology (2002) View source The rabbit study used whole fresh leaves.[3]Effect of tulsi (Ocimum Sanctum Linn.) on sperm count and reproductive hormones in male albino rabbits — International Journal of Ayurveda Research (2010) View source
These doses are far higher than anything in the anxiety and stress evidence, and a 2014 review argued the rabbit dose was high. It also reported that some European regulators restricted holy basil after the rabbit study.[4]Posological Considerations of Ocimum sanctum (Tulasi) as per Ayurvedic Science and Pharmaceutical Sciences — Indian Journal of Pharmaceutical Sciences (2014) View source
Pregnancy and Breastfeeding
There is no human evidence on holy basil in pregnancy either way. Given animal studies on implantation and ovulation, the sensible position is simple: avoid it while pregnant, breastfeeding or trying to conceive, and that includes strong daily tea.
- Human pregnancy trials: 0.
- Implantation: none in female rats mated with treated males.
- Ovulation: reduced in female rats on the extract.
- Breastfeeding: no data at all.
In the 2019 female-rat study the leaf extract raised progesterone, while pure eugenol raised both estradiol and progesterone.[5]A comparative study of eugenol and Ocimum sanctum Linn. leaf extract on the antifertility effect in female albino rats — Journal of the Chinese Medical Association (2019) View source
The implantation finding came from a 2009 study of male rats given the extract before mating.[6]Effect of benzene extract of Ocimum sanctum leaves on cauda epididymis of albino rats — Journal of Basic and Clinical Physiology and Pharmacology (2009) View source
Blood Sugar and Diabetes Medication
This is the caution with human data behind it. In a 1996 trial, holy basil leaves lowered fasting glucose in people with type 2 diabetes. On its own that is a possible benefit; on top of glucose-lowering medicine it is a possible problem.
- Insulin and sulfonylureas: the combination carries the most risk.
- Metformin and others: lower risk alone, but tell your prescriber.
- Signs of low sugar: shakiness, sweating, hunger, confusion.
- What to do: check readings more often for the first 2 weeks.
Fasting glucose fell 17.6% during the holy basil period compared with placebo leaves.[7]Randomized placebo-controlled, single blind trial of holy basil leaves in patients with noninsulin-dependent diabetes mellitus — International Journal of Clinical Pharmacology and Therapeutics (1996) View source
Liver, Kidneys and Other Blends
People also ask about the liver and kidneys. There is no human signal of harm to either, and there is also no long-term study that looked. The more practical risk is doubling your dose without noticing.
- Liver: no signal in human studies, no long-term data.
- Kidneys: no human study tested kidney function.
- Blends: calming formulas often contain holy basil already.
- Our Sweet Calm: contains holy basil, so count it as a source.
Stacking is also why the ashwagandha comparison matters, since the 2 are often sold in the same formula.
The Thyroid Question
Several pages state that holy basil lowers thyroid hormone. That claim traces to a single 1998 study in male mice, and it has not been tested in people. It is worth knowing, and it is worth knowing exactly how small it is.
- Animal: male mice.
- Dose: 0.5 g per kg of body weight, for 15 days.
- Result: lower T4 (thyroxine), no marked change in T3.
- Human data: none.
The authors described the extract as antithyroidic in mice.[8]Ocimum sanctum leaf extract in the regulation of thyroid function in the male mouse — Pharmacological Research (1998) View source Until a human study exists, the thyroid caution is a precaution rather than a finding.
How to Use It Safely
For a healthy adult who is not pregnant and takes no glucose-lowering or thyroid medicine, holy basil has a reassuring short-term record. These habits cover almost all of the practical risk.
- One source: not a capsule plus a blend plus daily tea.
- Tell your prescriber: if you take any regular medicine.
- Set a review point: 8 weeks, the longest trial length.
- Stop: 2 weeks before planned surgery, and if anything feels wrong.
Kept to one source, holy basil in a measured capsule makes your total easy to know.
Stop or Speak to a Clinician First If
- You are pregnant, breastfeeding or trying to conceive, where the only data comes from animals
- You take insulin, a sulfonylurea, metformin or any other glucose-lowering medicine
- You have a thyroid condition or take thyroid medication
- You have surgery planned within the next 2 weeks
- You already take a blend that contains holy basil, so you would be doubling it
Frequently Asked Questions
What are the potential side effects of taking holy basil? +
In human studies, nothing significant: the 2017 review of 24 studies found no significant adverse events. The cautions are interactions and animal findings.
Who should not use holy basil? +
People who are pregnant, breastfeeding or trying to conceive, anyone on glucose-lowering medicine without their prescriber's agreement, and anyone within 2 weeks of surgery.
What not to mix with holy basil? +
Glucose-lowering drugs such as insulin, sulfonylureas and metformin, because 1 trial found fasting glucose fell 17.6%. Also other blends that already contain holy basil.
Is holy basil hard on the kidneys? +
0 human studies we found reported kidney harm, and none tested kidney function directly. With kidney disease, ask your specialist before adding any herb.
Is holy basil hard on your liver? +
The 2017 review of 24 human studies reported no significant adverse events of any kind. Long-term liver data does not exist, so mention it if you have liver disease.
Does holy basil raise estrogen? +
No human data says so. 1 study in female rats found the leaf extract raised progesterone, not estradiol; pure eugenol raised both.
Is holy basil safe during pregnancy? +
Avoid it. 0 human trials exist, and rat studies reported effects on fertility and implantation.
Does holy basil affect fertility? +
In rats and rabbits, yes: lower sperm counts. 1 rat study found counts recovered 2 weeks after stopping. It has never been tested in people.
Does holy basil affect the thyroid? +
1 mouse study in 1998 found lower thyroxine after 15 days at 0.5 g per kg. No human study has tested it, so tell your clinician if you take thyroid medicine.
Should I stop holy basil before surgery? +
Stopping 2 weeks before a planned operation is a common precaution for herbs that can affect blood sugar. Follow your surgical team's instructions.
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