A single dose could block the effects of heroin for days
Long-acting implants of naltrexone could help heroin users to break their dependence. The technology has been around since the Eighties, so why have they yet to appear on our pharmacy shelves? Colin Brewer reports

Many of our most effective medical treatments were discovered by accident, but as our knowledge of the workings of human body grows, scientists are increasingly able to design drugs with a particular function in mind. In the Sixties, as illicit drug-taking became an epidemic, the US government encouraged research into drugs that would block the effects of heroin.

In 1965, scientists came up with naltrexone, a molecule that looked a bit like heroin but without its effects. Like a pharmacological cuckoo, it sits on the opiate receptors, preventing access by heroin molecules. Furthermore, a single dose could block the effects of heroin for up to three days. Having taken naltrexone, addicts can inject or smoke as much heroin as they want without it having the slightest effect.

The snag is that many addicts have mixed feelings about giving up heroin. Many believe - especially when they have not used it for a few months - that just one little smoke will not do any harm. So they stop taking naltrexone to do just that, or because they feel strong enough to abstain without it.

Naltrexone works best when some third party - parent, spouse or probation officer - supervises its administration as part of a formal or informal agreement. It is perfect for addicted doctors who are anxious to reassure their colleagues (or the GMC) that they have stopped helping themselves to heroin. But it is not much use to a friendless, homeless, unemployed addict with no one to supervise him or her and few reasons for abandoning the comforts of heroin.

Naturally enough, quite a few people - both heroin addicts and those trying to help them - have asked whether a long-acting injection or implant of naltrexone would be possible. Instead of having to decide every two or three days whether they were going to take it, they would have to do so only about once a month. In between injections, they would know that there was little point in taking heroin.

That is certainly the fervent desire of one of my patients, who resumed heroin use in his forties after a 20-year gap. He stays on naltrexone for several months at a time, supervised by his wife, but finds it difficult to resist the temptation to have an occasional puff. Predictably, this is followed by a feeling that since he has sinned, he might as well go the whole hog.

US researchers have been working on a naltrexone implant since at least the mid-Eighties. The technology exists. We already have long-acting depot contraceptives and depot antischizophrenic drugs. These usually consist of micro-crystals suspended in oil or small, semipermeable plastic rods incorporating the drug, which then leaches out slowly.

The only pharmacological requirement is that the drug should not need such a large daily dose that a chunk of plastic the size of a tennis ball would be required to accommodate it. Naltrexone fits this requirement. The standard daily dose of 50mg can be compressed into a 2mm cube. So why is it taking so long for the researchers and the manufacturers to put something on the pharmacy shelves?

There are several reasons. First, getting any new drug - or even a new formulation of an old one - on to the market involves much legal and governmental bureaucracy and testing. Pharmaceutical companies need to be very sure of their market before they will contemplate the enormous investment that is required.

Second, in the US, which is the main market for such a drug, there is much opposition to medical treatments for addiction. With few exceptions, drug and alcohol abuse groups regard members who take naltrexone or Antabuse as cheating somehow. A number of doctors take the same view because they see addicts as moral weaklings who should not be offered even temporary crutches, or - in the worst cases - as undeserving of valuable medical resources.

Furthermore, under the US health system, private hospital groups, which form the bulk of the addiction industry, make a lot of money from long- term in-patient treatment. They are naturally hostile to any treatments that threaten their profits.

Finally, counsellors and other non-medical people have traditionally played a major role in addiction treatment. Many feel threatened by medical treatments that could undermine their position.

The welfare of addicts is surely more important than the sensitivities of those who are trying to help them. So could not some enterprising British firm - funded, perhaps, by money from the confiscated assets of convicted drug dealers - get something going on a small scale? Compared with the risks that many heroin addicts take several times a day, the risks of even an experimental treatment are really rather small. In any case, naltrexone seems to be one of the least toxic substances around. Even a baby could swallow a bottle of it without coming to any serious harm.

Between 10 per cent and 20 per cent of an average group of young heroin addicts are likely to be dead within 10 years. One addict, whom I never met, telephoned me several times last year because she had heard about the implant and thought I might know where to obtain one. I told her to keep in touch, but the calls stopped coming because she had died as a result of her addiction.

The writer is a consultant psychiatrist.