Insomnia is a symptom resulting from a variety of different causes, including depression, ageing, jet lag and serious undiagnosed illness. Any effective treatment needs to tackle the root cause of the problem.

Over-the-counter (OTC) medications are believed to contain safe dosages of active ingredients and to have a low risk of complications. However, many OTC drugs use powerful agents that, if taken in large quantities, could be equal in strength to prescription-only medications.

Pharmacies stock a cornucopia of various sleep aids available without prescription. One class of OTC sleeping pills contain antihistamines, specifically, diphenhydramine. Widely used for allergies and motion sickness, antihistamines also have sedative properties. However, there’s a price to pay. Diphenhydramine, for example, can induce abnormal muscle spasms (acute dystonia) (Pharmacotherapy, 1994; 12: 492-6). Other antihistamine side-effects are blurred vision, constipation, diarrhoea, nausea and vomiting, dry mouth and general dehydration. To counter this latter effect, make sure you drink plenty of water.

Yet, paradoxically, antihistamines may also cause nervousness and insomnia, especially in the elderly and young children, groups known to be more sensitive to medications in general.

Asthmatics should steer clear, as antihistamines can aggravate the condition, and pregnant women should exercise caution as antihistamines appear to affect the behaviour of the fetus in the womb (J Dev Phisiol, 1991; 15: 33-44). Moreover, antihistamines interfere with lactation, and can find their way into breastmilk (Pediatrics, 1994; 93: 137-50).

Another class of sleeping pills uses the hormone melatonin, produced by the pineal gland at the base of the brain. Melatonin plays a role in regulating the sleep-wake cycle, but does not induce sleep per se (HK Pract, 1997; 19: 669-72). In the US, it’s classed as a food supplement and, thus, is unregulated by the Food and Drug Administration (FDA). In contrast, the UK’s Medicines Control Agency, part of the Department of Health, classifies it as a hormone, making melatonin a prescription-only medicine. However, it’s easily obtainable through the Internet.

Melatonin has a number of side-effects: the more common ones are diarrhoea, abdominal pain and headaches; less frequently reported are nightmares, morning grogginess, fuzziness, nausea, mild depression and low sex drive. It has a complex relationship with the body’s sex hormones - especially oestrogen and progesterone in women - and has been specifically linked with periodic depression and premenstrual syndrome (PMS) (Arch Gen Psychiatr, 1990; 47: 1139-46). A study of the effects of melatonin among night workers, who commonly suffer from insomnia, found adverse effects on both memory and performance (Chronobiol Int, 1993; 10: 315-20).

As melatonin can also interact unpredictably with other drugs, it should not be taken with immunosuppressants (such as azathioprine and cyclosporin), corticosteroids (such as prednisone), antianxiolytics, narcotic painkillers like codeine, sedatives, psychiatric medicines, antiseizure drugs, muscle relaxants or antihistamines such as diphenhydramine, or sedative herbs such as valerian or kava. Melatonin has caused seizures in children with no history of having them, and increases the number of episodes in those predisposed to them (Lancet, 1998; 351: 1254). With no standard recommended dosage, overdosing is easy: symptoms include headache, drowsiness and upset stomach.

In short, self-treat insomnia with OTC drugs only for short-term sleep difficulties. If you suffer from chronic insomnia, it is advisable to consult your physician or alternative practitioner.

Pharmaceutical sleep aids can prevent the restorative and therapeutic effects of sleep, while a few simple steps (see box above) can cure short-term insomnia. If symptoms persist, then OTC self-medication may be doing more harm than good.