But what's in the drugs?

I received an assortment of street drugs purchased by The Observer journalist, and I weighed the contents then analysed them for their drug content and purity.

These figures show that the amount you get for your money can vary considerably from one location to another, and a half-gram deal can contain as little as one-tenth of a gram. His haul in Edinburgh included a bag of heroin containing 0.04g - less than one-twentieth of a gram.

The purity also varied very considerably with some 'heroin' being mainly pulverised stone and glass. There is no way of knowing what the purity is when the drugs are being bought.

Casting the issue of legality to one side, drug users are being exploited in a way that no trading standards would allow if this were any other consumer commodity. What's more, using drugs of unknown quality is highly dangerous. It can easily lead to health problems or inadvertent overdosing.

If you're used to 'scoring' 'low purity' heroin, and you inject 90 per cent, it could prove fatal. The National Drugs Helpline put drug users in touch with us at SureScreen for any technical help, so we have to be hands-on professionals giving help that is easy to understand.

As a forensic scientist specialising in drug-related cases, I now feel that drug users should be encouraged to test their drugs. We are looking at making a little, automated device to test ecstasy tablets. We have shied away from this sensitive issue, but we now feel the home-testing market is ethically justified.

Those other elements in full

Heroin: Nutmeg, brick dust, stone, glass... anything from the dealer's yard, as long as it maintains a persuasive colour.

Ecstasy: Traditionally, with white tablets: food dye, starch, talcum powder. But now different colours have started to appear on the market in part, I imagine, because it allows the dealers to throw in all sorts of stuff. The white colour was at least some sort of protection.

Cocaine: Sugar, flour, talcum powder, starch, even calcium silicate (used on arm injuries). And, in terms of the "active" ingredient, increasingly you will now have cheaper amphetamines (speed) instead of cocaine.

· Jim Campbell is a forensic scientist at SureScreen Labs, Derbyshire