Rosemary is safe in the amounts typically used in cooking, but consuming large quantities of the herb or its concentrated oil can cause real harm. The problems range from stomach irritation at moderate excess to seizures, kidney damage, and fluid buildup in the lungs at truly high doses. The gap between “a few sprigs on your roasted chicken” and “dangerous” is wide, but it narrows quickly when concentrated extracts or essential oils are involved.
What Happens When You Have Too Much
Large amounts of rosemary can irritate the stomach and intestines, causing nausea and vomiting. Beyond that, excessive intake has been linked to kidney damage, seizures, coma, and pulmonary edema (fluid collecting in the lungs). These severe effects are associated with concentrated forms, not with sprinkling dried rosemary on food.
The compound responsible for much of rosemary’s toxicity at high doses is camphor, a naturally occurring chemical in the plant. Camphor is a neurotoxin whose molecular structure lets it cross from the bloodstream into the brain with ease. Once there, it disrupts normal nerve signaling and can trigger seizures. Rosemary also contains a compound called 1,8-cineole, which has similar convulsive properties. Together, these chemicals are what make rosemary essential oil potentially dangerous when swallowed or used in large amounts.
Seizure Risk Is Real
Rosemary has well-documented epileptogenic properties, meaning it can provoke seizures. This risk is most relevant if you have epilepsy or a history of seizures, but it applies to anyone exposed to enough camphor. In one striking case, a man with epilepsy who had been seizure-free for eight years experienced a breakthrough seizure after receiving a massage with a blend of oils that included rosemary. The camphor content in the rosemary oil was identified as the likely trigger. He did not have another seizure for at least a year afterward.
If you have a seizure disorder, avoid rosemary essential oil entirely, whether ingested, inhaled, or applied to the skin. Culinary amounts of the dried or fresh herb are a different story and are generally not concentrated enough to pose this risk.
Medications That Don’t Mix Well
Rosemary has mild diuretic effects, meaning it encourages your body to produce more urine. This becomes a problem if you take lithium, because increased urination changes how your kidneys process the drug, potentially allowing lithium to build up to toxic levels in your body.
The herb can also amplify or interfere with blood thinners (anticoagulants), blood pressure medications like ACE inhibitors, and diuretic drugs. Rosemary produces effects that overlap with these medications, so combining them can push the total effect beyond what’s safe. If you take any of these and you’re using rosemary supplements or drinking large amounts of rosemary tea daily, that’s a combination worth discussing with your prescriber.
Effects on Blood Sugar
Rosemary has measurable blood sugar-lowering properties. In one human study, participants taking 10 grams per day of dried rosemary leaf powder for eight weeks saw fasting blood sugar drop by about 18%. Even the lowest dose group (2 grams per day) experienced an 11% reduction. Lab studies show that rosemary extract can mimic some of insulin’s effects and block enzymes involved in sugar absorption by up to 60%.
For most people, this is a non-issue at culinary doses. But if you take diabetes medication, regularly consuming concentrated rosemary extract or large quantities of rosemary tea could push your blood sugar lower than expected. The risk is compounded if you’re already on drugs designed to do the same thing.
It Can Reduce Iron Absorption
Adding rosemary extract to food reduces how much iron your body absorbs from that meal. A study published in The American Journal of Clinical Nutrition found that rosemary extract decreased non-heme iron absorption (the type found in plant foods, beans, and fortified grains) by a statistically significant amount. If you’re already low in iron or prone to anemia, drinking rosemary tea with meals or regularly adding large amounts of the herb to iron-rich foods could make the problem worse over time. Spacing your rosemary intake away from iron-rich meals is a simple workaround.
Skin Reactions From Topical Use
Rosemary extract can cause allergic contact dermatitis in some people. Reported cases are rare, but they do occur. In one documented case, a 23-year-old woman developed itchy redness on her face after using a cleansing gel containing rosemary leaf extract. Patch testing confirmed that the rosemary extract itself was the allergen. If you notice irritation after using a skincare product with rosemary, check the ingredient list and consider patch testing a small area before continued use.
Pregnancy Concerns
Rosemary is traditionally classified as an emmenagogue, meaning it can stimulate menstrual bleeding. This has raised long-standing concerns about miscarriage risk. In practice, the evidence is nuanced: while essential oils from emmenagogic plants are generally considered unsafe during pregnancy because they could theoretically trigger uterine bleeding, there is no decisive evidence that rosemary oil at typical aromatherapy doses actually causes miscarriage. Still, concentrated rosemary supplements and essential oil are best avoided during pregnancy as a precaution. Cooking with rosemary in normal amounts is not considered a concern.
Where the Line Is
The core distinction is between culinary use and concentrated forms. A teaspoon of dried rosemary on your potatoes, a sprig in your soup, or an occasional cup of rosemary tea falls well within normal use and poses no meaningful risk for healthy adults. The danger zone starts with rosemary essential oil (which should never be swallowed undiluted), high-dose supplements, and consuming unusually large quantities of the herb daily over extended periods.
People most vulnerable to rosemary’s negative effects include those with epilepsy or seizure history, anyone taking lithium or blood thinners, people on diabetes medication, pregnant individuals, and those with iron deficiency. For everyone else, rosemary remains one of the safest and most commonly used herbs in the world, as long as you’re using it as a seasoning rather than treating it like a supplement.