Rosemary essential oil: chemotypes, hair claims and safe use

Rosemary essential oil is steam distilled from the leaves and flowering tops of the rosemary bush, Salvia rosmarinus. It smells fresh, green and sharply camphor-like, and most people use it in one of two ways: a few drops in a diffuser on a sluggish morning, or diluted in a scalp oil because of something they read about hair. It is an inexpensive, useful oil with a few real cautions. It is also one of the few oils where the exact bottle you pick up matters as much as how you use it, because "rosemary" comes in three chemical types that behave quite differently.

There is a rosemary bush by the steps of a house on my street, planted against a sunny wall, and it has shrugged off every typhoon season I have watched it through. That is the plant's nature. It comes from dry Mediterranean hillsides and likes sun, wind, poor soil and being left alone, which also makes it easy to keep in a pot on a Japanese balcony. Its older Japanese name is mannenro, though most people here simply call it rosemary.

Two names, one plant

From the eighteenth century onwards the plant was Rosmarinus officinalis. In 2017 botanists moved it into the sage genus, so newer labels say Salvia rosmarinus. Both names describe the same plant and the same oil, and plenty of careful suppliers still print the old one. Neither name tells you what matters most, which is the chemotype.

The three rosemary chemotypes

A chemotype (written "ct." on labels) is a chemical type within one species. Rosemary grown in different places, from different stock, gives oils in which different compounds dominate. These are the three you will meet:

If a label says only "rosemary", the oil inside could be any of these or a mix, though the cineole type is the commonest in home ranges. Ask the seller. A supplier who can name the chemotype and country, and show an analysis for the batch, is a better bet than one who cannot.

Your nose helps too. Put a drop of each type on a paper blotter strip and come back after ten minutes: the cineole type stays bright and clean, while the camphor type has a heavier, more medicinal edge.

How people use rosemary at home

Rosemary is a daytime oil in my house. These are the amounts I work with for healthy adults:

1. In a diffuser: 2 to 3 drops in a small ultrasonic diffuser (a 100 to 150 ml tank), run for 30 to 60 minutes and then switched off, with a window open a crack. It is a strong smell, and 1 drop of rosemary with 2 of sweet orange is friendlier than rosemary alone.
2. On a tissue: 1 drop, held a little way from the nose, for a stuffy commute or a long afternoon at a desk. Keep it away from your eyes.
3. In a body oil: 1 to 2 percent, which is 2 to 4 drops in 10 ml of carrier oil, for tired legs after a long walk or stiff shoulders after a day at the keyboard. For a small area over a few days you can go up to 3 percent (6 drops in 10 ml).
4. In a scalp oil: 1 to 2 percent in a light carrier such as jojoba. Part the hair, massage a little into the scalp with your fingertips for a few minutes, leave it for half an hour to a few hours, then shampoo. Once or twice a week is plenty.
5. In the bath: 3 drops mixed into a tablespoon of unscented shower gel or bath dispersant, added to the filled bath and swished. Never drip it straight in, since the oil floats and meets your skin undiluted. For a foot bath, 1 drop in a teaspoon of dispersant.

My dilution chart has the drop counts for other bottle sizes. I leave rosemary out of face blends: it stings near the eyes, and gentler oils do that job better.

Rosemary oil for hair: what the trial found

Most searches for rosemary oil now are about hair, thanks to short videos promising thicker hair within weeks. Almost all of them trace back to one study.

In 2015, researchers in Iran compared rosemary oil with minoxidil, the familiar over-the-counter hair loss treatment, at its lower 2 percent strength. About 100 men with male-pattern hair loss used one or the other on the scalp for six months. At three months neither group had changed much. At six months, hair counts had risen in both groups by a similar amount, and scalp itching was more common with minoxidil.

That is a genuine result, but look at its limits. There was no placebo group, so we cannot tell how much of the change came from the oil, from regular scalp massage or simply from time. It was one trial, in men only, with one kind of hair loss, and as far as I know nobody has yet repeated it on a larger scale. An older trial in Scotland, from the late 1990s, had people with alopecia areata (patchy hair loss) massage a blend of thyme, rosemary, lavender and cedarwood oils into the scalp every day, and found more improvement than with carrier oil alone. Again, a small study, and a blend, so it says little about rosemary by itself.

Two practical points come out of this. First, many "rosemary oil" hair products are infused oils: rosemary sprigs steeped in olive or another plant oil. These are far milder than the essential oil and a different product altogether. Second, never put the essential oil neat on your scalp, and be wary of hair recipes that use 10 or 20 drops in a tablespoon of carrier, which works out at roughly 3 to 7 percent. An irritated, flaky scalp does not grow better hair.

Hair loss also has causes no oil will touch. Low iron and thyroid problems are common culprits, and both show up on a simple blood test. See a doctor, or in Japan a hifuka (dermatology clinic), if hair comes out suddenly or in clumps, if you find round bald patches, if your scalp is sore, scaly or weeping, or if the hair loss comes with tiredness, weight change or heavy periods. The NHS has a plain guide to hair loss at nhs.uk if you would like to read more first.

Memory and focus

"There's rosemary, that's for remembrance," says Ophelia in Hamlet, and the idea has stuck. Researchers at a university in north-east England have run a series of small studies in which volunteers did memory tasks in a room scented with rosemary or in an unscented room. Some memory scores came out a little better with rosemary, and in one study people with more 1,8-cineole in their blood did better on some tasks. The effects were small, the sessions short and the settings artificial.

I think of rosemary in the study room the way I think of a cup of sencha (everyday green tea) at the desk: pleasant, perhaps a small nudge, but not a study method. That is fine for adults. It is not something to diffuse where young children are working or sleeping.

Who should leave rosemary alone

• Babies and young children. The cineole and camphor in rosemary can trigger breathing problems near a small child's face. Keep it off their skin and out of the rooms where they sleep; cautious writers extend this well into primary-school age. My piece on essential oils around children and pets has the detail.
• Epilepsy. Camphor and, in large amounts, cineole can provoke seizures. If you or your child has epilepsy, leave rosemary out, the camphor type above all, and tell the doctor what oils you use at home.
• Pregnancy and breastfeeding. Avoid the camphor type. Many aromatherapists leave rosemary out of pregnancy blends altogether, especially in the first three months, so ask your midwife. When breastfeeding, keep blends off the breast.
• Asthma. Sharp scents tighten some people's chests. Diffuse lightly or not at all, and stop if you start coughing or wheezing.
• Cats and dogs. The herb in the garden is not considered poisonous to them, but the oil is a concentrate, and cats in particular clear many of its compounds poorly. Never put it on a pet, keep the diffuser out of rooms where they eat and sleep, and leave a door open so they can walk away.
• Anyone tempted to swallow it. Rosemary is a kitchen herb; its essential oil is not food. Never add it to water, tea or capsules.

Fresh rosemary oil rarely sensitises skin, but old, oxidised oil irritates more. Patch test a new blend on your inner forearm and wait 24 hours before using it widely. A rash that spreads, blisters or will not settle is a reason to see a pharmacist or doctor. If oil gets in an eye, rinse with clean running water for at least 10 minutes. If a child swallows any, do not make them sick: phone a doctor or a poison information service at once, and call 119 in Japan (or your local emergency number) for drowsiness, coughing fits, trouble breathing or a seizure. The essential oil safety chart puts rosemary beside the other oils that need this kind of care.

Choosing a bottle

• The Latin name, Salvia rosmarinus or Rosmarinus officinalis, and ideally the chemotype.
• The country of origin and a batch number.
• Dark glass with a dropper insert, in a size you will finish: 10 ml lasts most households a year.
• A sensible price. Rosemary is one of the less costly oils, usually in the same range as true lavender. Much lower than that and it may have been stretched with eucalyptus or camphor fractions.
• Honest labelling on blends. A big bottle of ready-made "rosemary hair oil" is a carrier oil with a little essential oil in it. That is fine if the label gives the percentage, and a poor buy if it does not.

If you want to try it, a small bottle of rosemary essential oil is the sensible start. Write the opening date on the label and use it within about a year, because like most herb oils it loses its bright top note and turns harsher as it oxidises.

Oils that blend well with rosemary

• Lavender, to soften it: 2 drops of lavender to 1 of rosemary is a good starting point
• Sweet orange or distilled lemon, for a bright morning blend
• Peppermint, sparingly, for a sharp study-room mix
• Atlas cedarwood or frankincense, for something deeper and woodier (my frankincense profile has more on that one)
• Hinoki (Japanese cypress), which turns it into something like a forest path after rain

Before you open a new bottle of rosemary, check two things: the chemotype on the label, and who else in the house will be breathing it in.