Researchers have measured levels of melatonin in a number of over-the-counter supplements sold in Canada and found amounts of the hormone can vary dramatically from what's listed on the products' labels.

Melatonin is produced in the brain and occurs naturally in small quantities in some meats, grains, fruits and vegetables. Supplements containing melatonin are often taken by people in a bid to overcome sleep problems.

A study by the University of Guelph, published this week in the Journal of Clinical Sleep Medicine, tested 30 melatonin supplements produced under 16 brand names and sold in grocery and drug stores.

The researchers found 71 per cent of the products -- which were not identified by name -- did not meet label claims. Higher-than-listed amounts of melatonin ranged between 12 and 25 per cent on average, but some products had nearly five times the hormone they were said to contain.

One chewable tablet, for instance, contained more than eight milligrams of melatonin, a 478 per cent increase over the 1.5 milligrams shown on its packaging.

Others were found to have melatonin levels lower than what was listed.

One supplement, which also contained lavender, chamomile and lemon balm, had 83 per cent less melatonin than the three milligrams each capsule was supposed to contain.

Principal investigator Praveen Saxena, a professor in the Ontario university's department of plant agriculture, said amounts of melatonin in the supplements tested sometimes also varied from lot to lot within the same product line.

Of even more concern was that about a quarter of the supplements also contained serotonin, a neurotransmitter involved in mood regulation and nerve impulse transmission, which was not included in the products' ingredients lists and could cause adverse effects.

"Millions of people use melatonin for a variety of purposes, including as a sleep aid and for jet lag," said Saxena. "These products are often self-prescribed, so it's important that labels are accurate and the products free from contaminants."

Serotonin is a regulated substance that's not authorized for sale as a supplement. Ingesting excess amounts of the neurotransmitter could lead to side-effects, including what's known as serotonin syndrome -- a potentially life-threatening condition. People who take antidepressants called selective serotonin re-uptake inhibitors, or SSRIs, might be susceptible to adverse effects if they were to take a melatonin supplement that also contained serotonin.

Dr. Brian Murray, a neurologist at Toronto's Sunnybrook Health Sciences Centre who specializes in sleep disorders, said the study's finding of huge variability in melatonin levels in supplements is concerning.

"Many people are using it, and if they are using it, who knows what they're getting?" said Murray, who was not involved in the study.

He does not recommend taking melatonin for sleep issues such as insomnia. Evidence shows the hormone may have a role to play in resetting a person's internal circadian rhythms, or sleep-wake cycle, but it's "not a good sleeping pill."

"It's not going to make you sleep more, it's not going to make you sleep deeper, but it might change the time when you have that sleep."

Melatonin can also interact with a number of prescription medications -- including antidepressants, blood pressure regulators and blood thinners -- and should not be taken without first checking with a health-care provider.

Saxena said melatonin, which occurs naturally in some plants, is an unstable molecule and in supplement form may be affected by environmental conditions.

"For example, manufacturers make them and then they are transported in trucks -- we don't know what the temperature was there and the storage conditions."

Differences among batches or improper mixing might also cause variability in melatonin content, although it's possible that extra melatonin was added during manufacturing to ensure a longer shelf life, he speculated.

"We wanted to help bring the point forward and not point fingers because we don't really believe that it is intentional," he said.

"I think it's more like this is a finding that can help consumers and help manufacturers improve production quality."

Helen Long, president of the Canadian Health Food Association (CHFA), said members strive to ensure consumers have access to natural health products that are safe, effective and of high quality.

"The published results do not adequately explain the cause of the reported variability," Long said in a statement, "and we would welcome further research to understand the external factors that could lead to variability or instability of melatonin."