The majority of the adult population in the UK takes drugs. This statement is surprising, but none the less true. A drug is defined as ?any substance whose action is to modify a chemical process in the body. If we accept this definition, then almost anything that we ingest, inject, inhale or otherwise allow to enter our body can be considered to be a drug.

If you still need convincing, consider the Pro-Plus pills to help that late night exam revision, or the aspirin to aid recovery from the excesses of the night before. The list of everyday drugs goes on - coffee, tea, alcohol, over-the-counter and prescription medication, and even chocolate.

There is nothing new about this ?universal drug habit?. Throughout history, people have used a number of different types of drugs, both naturally occurring and synthetically produced, to alter their mental or physical state.

If drug taking plays such a part in our everyday lives, why are drugs considered to be such a social problem?

The reality is that any substance, whether considered a ?beneficial? substance (e.g. analgesic medication or coffee) or a ?bad? substance (e.g. LSD or cannabis) is dangerous if abused. The potential danger is not only to the individual concerned, but also to people in close proximity, particularly young dependants, and to the community as a whole. The response by the UK Government to the problem of drug misuse has been the introduction of legislation, both to control the manufacture and supply of medicinal drugs (Medicines Act 1968), and to prevent the misuse of non-medicinal or illegal drugs (Misuse of Drugs Act 1971).

Recent figures published by the Office of National Statistics suggest a conservative figure of 320,000 known problem drug users in the UK. In addition, there are 1.8 million adults whose alcohol consumption is at a harmful level. The financial cost of dealing with substance abuse is estimated at ?30 billion a year, which takes account of loss of productivity at work, drug- associated criminal activity, policing and the costs to the Health Service. All these factors have led to changes in governmental policy and a more proactive approach towards the treatment and rehabilitation of problem drug users.

However, before any form of treatment can begin, it is important to ascertain exactly what drug or drugs are being misused, at what levels and over what time period. This is now possible, due to advances in analytical science.

When ingested, smoked or injected, drugs enter the bloodstream and are converted into specific metabolites. These metabolites circulate throughout the body, where they are incorporated into keratinised matrices ? the hair and nails ? or excreted via urine or sweat. The presence of one or more of these specific drug metabolites in a sample of blood, urine, sweat or hair taken from the subject is conclusive evidence of drug misuse.

The choice of sample to be analysed will depend on the period of time over which any drug use needs to be detected. Each type of drug test has specific window of detection, ranging from just a few days to many months, depending on the sample that is chosen for analysis. If, for example, you wanted to establish that a drug was consumed within the last few days, then urine or blood analysis would be the best choice, as the window of detection is between 3 and 6 days. However, if chronic misuse needs to be established, a hair drug test would be the most appropriate, as drug testing of hair can establish the pattern of substance abuse over a period of 6 months or more. Of course, other factors such as the possibility of adulteration of the sample, storage of the sample, ease of collection and expense also need to be taken into consideration.

To some extent, the accuracy and sophistication of drug testing techniques have overtaken the public's acceptance of its use. Society does not, at present, endorse widespread drug screening programmes. However, in those groups where it has been deemed acceptable to carry out drug screening ? prisoners, airline pilots and members of the armed forces ? it has proved an effective means not only of detecting substance misuse, but also of monitoring and supporting drug rehabilitation programmes.