What is Kaffir Lime?

Kaffir lime (Citrus hystrix) is a small, bumpy green citrus whose fragrant leaves and peel are used in cooking and in home remedies. People use it mainly as a food flavouring, a scented oil, and a traditional hair and scalp rinse.

The honest verdict: almost all of the research is laboratory and animal work. Long traditional use tells you that people found a plant useful, but it is not proof that it treats any illness, and you should read everything below with that in mind.

In Western and European kitchens the fruit is a recent import rather than an old herbal staple. Used as a food or a diluted scent it is generally well tolerated. The concentrated essential oil is the part that needs real care, because citrus peel oils can irritate skin and make it burn more easily in sunlight.

How does Kaffir Lime work?

No one has shown how kaffir lime works in the human body, because the human studies have not been done.

In test tubes, the peel and leaf oils slow the growth of some bacteria and fungi. That effect is thought to come from the oil dissolving into microbe cell walls. Whether anything similar happens on your skin or scalp at safe, diluted strengths is unknown.

The fruit juice is sour and acidic, much like other limes, which is why it has been used as a cleansing rinse. That is a simple physical effect, not a medicinal one.

What forms does Kaffir Lime come in?

Fresh or frozen leaves are the most common form. They are torn or shredded into soups and curries and are treated as a food, not a supplement.

Dried leaves and dried peel are sold for cooking and for making infusions. Drying weakens the aroma, so more is usually needed for the same flavour.

The fruit juice is very sour and is used mostly as a scalp and hair rinse, diluted with water. It is rarely drunk on its own.

Essential oil, steam-distilled from leaf or peel, is the most concentrated form. It is sold for scent and for diluted skin use, and it should never be swallowed or put on skin undiluted.

Where it comes from:

Kaffir lime comes from tropical Southeast Asia, where it has been grown for centuries in Thailand, Indonesia, Malaysia and the Philippines. Its leaves are a staple of the region's cooking and its juice has long been used as a hair and scalp wash.

It reached Western and European kitchens mainly in the late twentieth century, alongside the rise of Thai food. There is no old European herbal record for it.

Note on the name: many people now prefer makrut lime, because the older name carries a racial slur in parts of southern Africa.

What does Kaffir Lime help with?

• Dandruff and itchy scalp (traditional rinse): This is the best-known traditional use, especially in Southeast Asia, where diluted juice is massaged in and rinsed out. There are no published human trials testing it for dandruff. Any benefit may come simply from the acidity and the wash itself. Stop if your scalp stings, reddens or flakes more.
• Hair and scalp cleansing: Used traditionally as a clarifying rinse to cut oil and leave hair smelling fresh. This is a grooming use rather than a medical one, and it has not been studied in people. Very sour juice used often and undiluted can dry hair and irritate the scalp.
• Head colds and blocked nose (steam or aroma): Leaves are sometimes added to hot water and the steam breathed in. Warm steam itself can loosen mucus and make you feel clearer for a short while. There is no evidence that kaffir lime adds anything beyond the steam and the smell, and no evidence it shortens a cold. Keep your face far enough from the bowl to avoid scalding.
• Digestive upset and nausea (culinary use): Sour, aromatic citrus is a long-standing kitchen answer to a heavy or greasy meal. This rests on food tradition, not on trials. If nausea is new, severe or lasts more than a couple of days, that needs a doctor rather than a herb.
• Minor skin blemishes (traditional topical use): Lab tests show the oils slow the growth of some skin bacteria and fungi in a dish. That has not been tested on human skin. Undiluted oil or juice on the face can cause irritation and sun-related burns, so this use carries more risk than most people expect.
• Insect repellence (laboratory work only): Kaffir lime oil has shown activity against mosquito larvae and some insects in laboratory studies. No human field trials have compared it with standard repellents. Do not rely on it where mosquito-borne illness such as malaria or dengue is a real risk.

Which foods are rich in Kaffir Lime?

1. Thai tom yum and tom kha soups: shredded leaves added near the end of cooking
2. Green and red Thai curries: leaves simmered in coconut milk
3. Indonesian and Malaysian rendang and sambal: leaves and zest for aroma
4. Fish and seafood dishes: the leaf cuts a strong fishy taste
5. Infused syrups, teas and cordials: leaves steeped in hot water or sugar syrup

Practical Tips:

The leaf is tough and fibrous, so shred it finely or lift out whole leaves before serving.

What are the side effects of Kaffir Lime?

• Skin irritation, stinging or redness from juice or undiluted oil
• Photosensitivity (skin burning more easily in sunlight) after the oil is applied
• Allergic skin reaction, more likely if you react to other citrus
• Dry or irritated scalp if the rinse is used too often or too strong
• Eye pain and watering if juice or oil gets in the eye
• Mouth or throat burning if concentrated oil is swallowed

Good to know:

Eaten as food, kaffir lime rarely causes problems and is treated as a normal ingredient.

Almost every reported problem involves the concentrated essential oil or neat juice on skin. Citrus peel oils are a well-known cause of contact irritation and of sun-related burns on treated skin.

Swallowing essential oil is the one genuinely dangerous use. Even small amounts can burn the mouth and throat and upset the stomach, and children are at much greater risk than adults.

How much Kaffir Lime should you take?

• Recommended daily: There is no established medicinal dose, because the human studies needed to set one have not been done. As food: two to four fresh leaves flavour a family-sized curry or soup. This is a normal food amount and needs no limit beyond taste. As a scalp rinse: the juice of one fruit mixed into a cup or more of water, left a few minutes, then rinsed out. As essential oil on skin: dilute to 1 to 2 percent, which is about 1 to 2 drops in a teaspoon of carrier oil such as almond or sunflower. Never swallow the essential oil.
• Best time of day: Any time as a food. Use the scalp rinse when you wash your hair. Apply diluted oil to skin in the evening, so the treated skin is not in strong sun afterwards.
• With or without food: Not applicable in the usual sense. The leaves and peel are eaten as part of a meal. Topical and inhaled uses have nothing to do with food.
• Time to effect: A steam inhalation or a scented rinse feels pleasant within minutes, but that is comfort rather than treatment. No meaningful time to effect can be given for any health claim, because none has been demonstrated in people.
• How long to take it: Culinary use has no time limit. For a scalp rinse, once or twice a week is plenty. Give it about four weeks, and if your scalp is no better, stop and see a pharmacist or doctor. Stop straight away at any sign of stinging, rash or increased flaking.