The government is coming under pressure to review the classification of certain controlled drugs, such as ecstasy and cannabis.

A forthcoming report by the Police Foundation is believed to recommend ecstasy should be downgraded from its class A status.

But, as it stands, which substances carry which penalties for users - and why.

Substances are controlled primarily by the Misuse of Drugs Act 1971, and the Medicines Act 1986.

The former divides the substances into three classes, A, B or C. Class A drugs are considered the most dangerous, class C the least.

The latter places substances within one of five medical schedules, which determine whether a drug has any perceived therapeutic use, and whether or not it should be prescribed. Schedule 1 is the most strictly controlled.

Drugs have been outlawed for different reasons and at different times.

Sometimes the reasons are medical, but just as often they are social.

Similarly, sentences given for supply or possession of the drugs vary according to many different factors, but broadly reflect society's perception of each substance at the time.

Class A

Class A drugs include cocaine and crack; ecstasy; heroin, morphine and opium; LSD; psilocin (as found in magic mushrooms), and - following a 1994 court case - cannabis oil.

The maximum penalty for posession is seven years imprisonment or an unlimited fine or both, and for supply, life imprisonment or an unlimited fine or both.

Cocaine was first banned in the UK under emergency laws during WWI, after rumours it was affecting soldiers' performance.

Heroin was outlawed at the turn of the last century, after having been developed to treat morphine and opium addiction - banned for their association with scandalous opium dens.

Today it is one of the drugs most frowned upon by society and the courts, and most likely to carry stringent sentences.

Cocaine and heroin can still be prescribed under licence from the Home Office for various treatments - but not to addicts.

Ecstasy was banned in 1977, after the government became alarmed by the spread of its recreational use.

LSD was outlawed in 1973, in reaction to its widespread use by "hippies" and other counter-culture groups.

Tiny amounts of it can be stored under license, for scientific research.

Class B

Class B drugs include amphetamines, barbiturates, cannabis and cannabis resin.

All class B drugs become class A drugs if they are in injectable form.

Maximum penalties are five years imprisonment or an unlimited fine or both for possession, and 14 years imprisonment or an unlimited fine or both for supply.

Amphetamines were banned in 1964 after a growth in recreational use, particularly among teenage mods.

Recreational use of cannabis was first prohibited in 1928, when it was linked with heroin and opium.

However, there are signs that society is relaxing its views. Debates on softening the law on drugs usually focuses on cannabis.

Class C and others

Class C drugs include various tranquillisers and mild, amphetamine type stimulants.

Maximum penalties are two years imprisonment or an unlimited fine or both for possession, and five years imprisonment or an unlimited fine or both for supply.

In recent years, ketamine - developed as an animal and human anaesthetic - has become a recreational drug among the post-rave generation.

It is not controlled under the Misuse of Drugs Act and therefore its possession is not a criminal offence.

However, the Medicines Act has it as a prescription only drug, which means its unauthorised supply is illegal.

The legal status of magic mushrooms is complex. The fungi themselves are not controlled and it is not unlawful to eat them.

But the active ingredients psilocin and psilocybin are considered Class A. So if mushrooms are prepared in any way, it is deemed to be a preparation of class A drugs and therefore unlawful.

The use of solvents, propellants and gases for intoxication is not controlled by law.

However, they are controlled by the Intoxicating Substances Supply Act 1985 in England and Wales, which is aimed at retailers.

Amyl nitrate is classed as a prescription only medicine under the Medicines Act.

Possession is not illegal, bit it can be an offence to manufacture, import, sell or supply it in the course of a business.