Molecule of the Month: Partake of the poppy to ease the pain: Opiates can provide optimum relief for those whose suffering is severe. John Emsley reports

This month the Independent highlighted drug abuse in Britain, of which heroin is the most worrying. Addicts can kill themselves by injecting too much; in Glasgow alone more than 100 young people died this way last year. Heroin for street sale is smuggled into the UK, but it is available legitimately as the painkiller diamorphine.

Heroin is one of a group of pharmaceuticals derived from the opium poppy and known as opiates. Along with morphine, it is used to relieve the pain of deep wounds or terminal cancers. Some opiates are available without prescription: codeine for headaches, dihydrocodeine for backaches, and pholcodine for coughs. These medicines are better known as their commercial preparations Solpadeine, Paramol and Tixylix.

Opium originated in Turkey more than 5,000 years ago and was prescribed by Hippocrates. It was the only effective painkiller available until the late 19th century, and was used as a solution in alcohol called laudanum. More than 20 chemicals have been separated from opium, but the most active molecules are morphine, which makes up to 7 per cent, and codeine, up to 2 per cent. Pure morphine was first extracted by a German chemist called Serturner in 1803. It is 10 times more potent than codeine, but heroin, first made in Germany in 1874, is 10 times more effective again. A little goes a long way, and it is easy to smuggle.

The managing director of the world's largest producer of opiates - and Britain's sole manufacturer - Dr Marshall Smalley, explains their popularity: 'Morphine is one of the most powerful painkillers a doctor can prescribe. The treatment of severe pain with morphine does not lead to addiction, but does improve healing rates. Doctors now recognise this, and worldwide sales of morphine have increased fourfold in the past eight years, particularly with the introduction of controlled-release tablets.'

More than two-thirds of the morphine extracted from opium is turned into codeine, with the rest converted to other opiates, including heroin, still the final defence against pain for terminal cancer patients. Dr Smalley says legitimate production of all opiates is 240 tonnes a year worldwide, of which 100 comes from Indian opium, and 140 from the alternative method of production, which uses poppy straw.

The traditional way of gathering opium is to make cuts in the unripe seed capsules. A milky juice then exudes, which dries and can be scraped off. For mechanical harvesting, the poppy crop is allowed to dry out in the field and harvested like wheat. The straw is crushed, the seeds removed (and sold to bakers for sprinkling on bread rolls) and the straw extracted with solvents to yield morphine.

The ideal climate is one with plenty of sunshine and dry summers. A heavy storm can ruin a crop of opium because it washes out the morphine. Most opium poppies used in the UK are grown in Tasmania, which produces a third of the world's legal crop.

Morphine works by acting on the central nervous system, blocking receptors, called opioids, which register the sensation of pain. By a curious quirk of nature the shape of the molecule resembles the body's own painkillers, the peptide endorphins, and like them can bind to the opioid receptors. It also soothes the emotional upset as well, and yet does not affect alertness.

There are side-effects with morphine and diamorphine. A minor one is constipation, and even this has been put to use: kaolin and morphine mixture is a treatment for diarrhoea. More worrying is the risk of addiction. This is why morphine is normally prescribed for only a few days after a serious operation, and diamorphine reserved for terminal cancer patients in their last months of life.

Chemists can modify morphine. The molecule has two alcohol groups to which other atoms can be attached. Most chemical changes reduce its activity. Add a methyl group and you produce codeine, a less effective painkiller because it does not fit the opioid receptor as well - but it also becomes much less addictive. The same is true of dihydrocodeine, which has two extra hydrogens, and pholcodine, which has a bulky morpholinoethyl group. These can be sold over the counter.

Morphine can be boosted chemically, but the danger then is more rapid addiction. When two acetyl groups are attached, to give heroin, it can penetrate the brain more easily, and once there it quickly converts to morphine.

In some instances an increased painkilling ability may be accompanied by lower addictive tendencies. This is the case with hydromorphone, the preferred opiate in US hospitals. It is made by moving a hydrogen atom from one of the alcohol groups to a carbon atom.

Some changes to morphine have unexpected effects. When the nitrogen atom in the molecule is tampered with, the product can be a morphine antagonist; in other words, it will expel morphine for the opioid receptors. However, unlike morphine, it provides no relief from pain. Naloxone is such a molecule, and is an instant antidote for people poisoned by morphine and heroin overdoses. It works within seconds.

The potency of an opiate can be judged by the maximum safe single dose, which for codeine and dihydrocodeine is 60mg, for morphine 20mg, diamorphine 10mg and hydromorphone 2mg. These are oral doses - injecting the drug requires much less. Pholcodine can also be taken at the rate of 60mg per day, provided it is taken in several doses.

Most opiates are not very soluble in water, and this protects against abuse. Unfortunately, heroin is extremely soluble, which is why injecting heroin that has not been properly 'cut' can be fatal.

The author is science writer in residence in the Department of Chemistry at Imperial College, London.

(Photograph omitted)