DOES IT WORK?:PEPPERMINT GETS its name from Mintha, a nymph in Greek and Roman mythology who was changed into a plant. Spearmint has been used medicinally since ancient times, writes DONAL O'MATHUNA
Peppermint was first identified in 1696 as a natural hybrid formed from the crossing of spearmint and water mint. This quickly led to peppermint being widely cultivated for its oil, especially in the US and Canada.
The largest market for peppermint oil is the chewing gum industry, followed by toothpaste makers and then pharmaceuticals.
Evidence from studies
Peppermint oil has been studied in detail in the laboratory. The main active ingredient is menthol (up to 50 per cent of the oil), but many other components have effects also. The main chemical difference between spearmint and peppermint oils is that menthol is absent from spearmint.
Menthol has a complex effect on sensory nerves, both stimulating the cold sensors and blocking pain receptors. This has led to its widespread use in topical preparations to relieve pain and itching.
But peppermint oil is also used orally where it has been found to relax smooth muscle by interfering with the role of calcium in muscle contraction.
A number of relatively small controlled trials have been conducted with peppermint oil capsules. Most of these found significantly greater relief from heartburn in those taking peppermint.
But a number of controlled studies in patients with other conditions found that those taking peppermint reported increased heartburn.
For this reason, a number of enteric-coated formulations of peppermint oil have been developed. This coating slows the release of the oil to minimise its effects on the upper digestive tract.
A small number of studies have used enteric-coated capsules and shown they relieve heartburn and intestinal cramps.
Some of these studies used capsules containing a mixture of peppermint oil and caraway oil, though it is unclear if the caraway oil is required for the beneficial effects.
A number of randomised controlled trials have also been conducted in patients with irritable bowel syndrome.
This condition does not have a satisfactory treatment, although various dietary and lifestyle changes can be helpful.
A systematic review of peppermint oil for irritable bowel syndrome was published last year in the British Medical Journal.
This found a small number of high-quality studies, but there was clear benefit for those taking peppermint oil.
An earlier review had found that out of 12 good-quality studies, eight showed that peppermint was more effective than placebo. A small number of trials have compared peppermint to conventional treatments and found them to have equivalent results.
Problematic aspects
Adverse effects are rare, but while peppermint oil is used to treat digestive problems it can cause them in some people.
Heartburn is the most common symptom, especially in anyone with chronic heartburn due to gastric reflux problems. Allergic reactions have been reported, and anyone allergic to any type of mint plant should use peppermint oil carefully.
Recommendations
Peppermint oil has a long history of use for digestive problems. Controlled trials have recently provided support for its use in treating heartburn and irritable bowel syndrome.
The latter is a welcome finding given the difficulty finding effective treatments for this condition. Newer pharmaceuticals tend to be expensive, and some have been withdrawn due to serious adverse effects.
While the latest results are highly encouraging, they have not been completely consistent. This may have to do with variability in the amount of menthol contained in various products, or may reflect differences between people’s sensitivity to peppermint.
For digestive problems, 0.2-0.4 ml of peppermint oil is usually recommended three times daily.
Given the frequency with which peppermint causes heartburn, enteric-coated capsules should be used if at all possible.
Dónal O’Mathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University