The government has announced plans to equip every police station in England, Wales and Scotland with the technology to test drivers for drugs more quickly and easily.

However, testing for drugs is much more complicated than testing for alcohol and presents a challenge for police and scientists alike.

What impact do drugs have on the body?

Studies on the effect of drugs on driving ability are limited.

According to the British Medical Association, experimental studies suggest high doses of amphetamines increase risk-taking and dangerous driving behaviour - such as speeding.

Studies on the influence of cocaine indicate drivers are impaired when they have taken the drug in high doses and during withdrawal periods.

It is thought by the government that cannabis impairs co-ordination, visual perception, tracking and vigilance.

Studies report that the majority of fatal cases with detected levels of cannabinoids are confounded by alcohol. It seems alcohol in combination with cannabis increases impairment and causes more crashes.

Ecstasy, meanwhile, is thought to cause blurred vision and poor judgement, while anti-depressant drugs can lead to slow reactions and an inability to maintain concentration. Legal, prescription medications can also have a negative impact on driving, and combining several drugs or mixing them with alcohol complicates their effects.

What can police currently do to spot drug-drivers?

At present, suspected drug-drivers can be asked to a roadside Field Impairment Test or FIT. This can include the Romberg Test in which they are asked to close their eyes and estimate when 30 seconds have elapsed - a challenge because drugs impair the body's internal clock. Drivers may also be asked to balance on one leg, touch their nose with the tip of their finger and walk heel-to-toe along a straight line. Police can also look for physical signs like dilated pupils.

If these tests do suggest drug impairment, the driver will be arrested and taken to a police station. They must then be examined by a doctor who determines whether they have a "condition which might be due to a drug". If they are deemed to, a blood test will be done to give definitive evidence.

How dangerous is drug-driving?

The Department for Transport (DfT) estimates that one in five drivers or riders killed in road accidents may have an impairing drug - legal or illegal - in their system. In 2008, drugs were reported as a contributory factor in 60 road deaths - approximately 3% of all fatalities - 280 serious injuries and 745 slight injuries, but the DfT says this is likely to be an underestimate.

What is the law - and penalty - at present?

Anyone caught over the legal alcohol limit can be prosecuted on that fact alone. However, the law for drugs is different. Police must have evidence that a person's driving has been impaired by the drugs in their system. Katie Shephard, from road safety charity Brake, says: "It's ridiculous. Every drug is different and everybody responds to drugs differently, so proving impairment is very difficult."

Anyone convicted of drug-driving faces a minimum 12-month driving ban, up to six months in prison and a maximum fine of £5,000.

Why is it more difficult to test for drugs than alcohol?

Because of the sheer number of substances involved. Kevin Delaney, from the Institute of Advanced Motorists (IAM), says: "Unlike alcohol, which is just one drug, the number and range of drugs that can impair somebody's driving is almost incalculable, and actually designing a piece of machinery that will deal with all of them is going to prove extremely difficult."

The Home Office has been working for at least 10 years on a roadside drug screening device, but they say that as well as the problem of substance diversity, it has proved difficult to devise a small, easy to use device that can withstand all weather conditions. However, campaign groups like Brake say the government should stop trying to develop a cover-all machine and start using some of those already in existence that can identify at least some drugs.

What is the government now working on?

The government hopes to develop a device for use inside police stations that could test a driver's saliva or sweat for drugs. The idea is that this would remove the need for a doctor to be involved - a step which currently slows the process considerably and means that by the time a blood test is administered, the level of the drug in the suspect's system may have dropped significantly.

A positive preliminary test would then be followed by a definitive blood test. Mr Delaney, from the IAM, says: "It will simplify the process, but it isn't going to revolutionise the process of dealing with drug-driving in the same way that the breathalyser did."

The government-commissioned North Review, which reported last month, recommended such a device, but also stressed that FITs must be more widely used. During the 2009 Christmas period, police conducted more than 223,000 alcohol breath tests compared with fewer than 500 FITs. It points out that as long as police must still have reasonable suspicion to arrest a driver and take him back to the station for a saliva test, FIT will remain important.

What do other countries do?

Roadside "drugalysers" have been used for some time in a number of countries, and several UK-based companies produce testing devices. Concateno, which supplies to police in countries including Australia, Spain and Italy, says it allows police to detect six different drugs in a person's saliva within five minutes. The company also says that since the devices were introduced in Australia in 2004, incidences of drug-driving have fallen from one driver in 44 to one driver in 76. Concateno spokesman Iain Forcer said: "This technology, used in conjunction with a widespread educational campaign, has and continues to be extremely successful."

The North Report did, however, conclude that in Sweden, where roadside drug testing is common, police have been "unsuccessful" in deterring repeat offenders.

Does the law need to change?

Brake and others want the law to be changed to remove the need to prove impairment. They also say that trying to come up with a set legal limit for the entire population for each kind of drug is so fraught with difficulty - and somewhat meaningless when the substance itself is illegal - that zero tolerance would make more sense.

The North Report, however, did recommend the introduction of drug limits and said research must begin in earnest to work out what those should be for each particular substance.

Nevertheless, it noted that in Western Australia, where zero tolerance has been adopted, it has proved to be "simple, straightforward, relatively quick to administer, and unambiguous".