The UK has put in place some of the strictest drug driving laws on the planet in an effort to get drug-impaired drivers off the roads. Breath screening and blood tests will be used to detect eight illicit drugs at "zero tolerance" levels, and eight further prescription drugs at levels that would begin to impair driving. Naturally, since the British government can’t be seen to encourage recreational drug use, these limits haven’t been put into a practical context. So we contacted several drug testing experts and a forensic pharmacologist to try to work out what they mean. And as it turns out, some drugs will make you illegal to drive long after their physical effects have worn off.

Driving limits for 16 different drugs have been approved in the UK and a testing process is now under development. The new limits tighten up and work in tandem with the existing system, in which police need to prove a driver is impaired through a roadside co-ordination test, and then prove that impairment was due to drug use.

How drivers will be tested

Drug screening will be done initially using Drugalyser oral swab tests, which test for chemical compounds in the saliva – and it appears that a positive saliva sample could lead to a blood sample test, taken at a hospital: “The police will use the new offence to prosecute drivers whose evidential blood samples contain more than the specified limits of those drugs provided for in regulations.”

UK drug driving penalties

Penalties will be stiff – up to six months in prison, up to £5,000 in fines and a license disqualification for at least 12 months. They're designed to be in line with drink driving penalties, and not to punish people for having illegal drugs in their system, which is not illegal in itself.

So, what do these limits mean for the average British drug user? The main takeaway as far as illegal drugs go is that the limits are extremely low – well below the point at which your driving would be impaired – and it’s possible to swab positive for several days after a high has worn off.

The UK government makes no apologies for this, calling it a "zero tolerance" approach rather than a "road safety" approach. The illicit drug limits have been set at the lowest possible level that rules out accidental exposure. For example, passive inhalation of marijuana smoke at a party.

How long before you're safe to drive?

So if you take drugs, how long before you’re safe to take a Drugalyser test? There’s no official advice on this. In fact, one forensic toxicologist told us “it would be irresponsible” to try to estimate when it would be safe to drive and that his best advice was “if you intend to drive, don’t do any drugs.”

The fact is, people take drugs, legal and illegal ones, in every part of the world. As somebody who recently lost a very close friend to a driver who tested positive for methamphetamine, I support any means to deter drug impaired driving. But as a realist, I think it’s important to give recreational drug users the information they need to act responsibly.

So let’s look at each illicit drug individually. But before we start, it’s important to understand that each of these figures can be affected by the size of the dose, the body mass and physical composition of the user, the user’s past usage habits, the effects of mixing drugs including alcohol, and a range of other factors that may increase or decrease detection times.

Illicit drugs

“Zero tolerance” limits

Benzoylecgonine, 50 µg/L, Cocaine, 10 µg/L

These are grouped together because benzoylecgonine is a cocaine metabolite that stays in the system slightly longer than the cocaine itself. Cocaine and its metabolite can be detected in saliva for between 2-5 days after use.

Delta–9–Tetrahydrocannabinol (Cannabis and Cannabinol), 2 µg/L

A 2-microgram limit for THC is extremely low. Marijuana use can be detected in the saliva for up to 24 hours after exposure. But it’s worth noting that it lasts much, much longer in the urine and blood. Habitual heavy users can test positive after as much as a month without the drug, as it is stored in body fat and re-released into the bloodstream when the fat is burned.

The roadside Drugalyser test will most likely only detect THC use within the last 24 hours, but any subsequent blood or urine testing may show results from 2 days to 28 days later in chronic heavy users.

Ketamine, 20 µg/L

It’s difficult to find information on how long Ketamine lasts in the system. It’s detectable in urine for up to 2 weeks, but seems to last only around 3 days in the saliva.

Lysergic Acid Diethylamide (LSD), 1 µg/L

LSD can be detected in saliva for 1-2 days after use.

Methylamphetamine - 10 µg/L

Methamphetamine can be detected in the saliva for 1-3 days after use. It’s noted in public consultation that certain types of amphetamines are used in the medical treatment of ADHD and other conditions. It’s unclear at this stage what provisions will be made for legal amphetamine users.

Methylenedioxymethaphetamine (MDMA – Ecstasy), 10 µg/L

MDMA may be detectable in the saliva for as much as 1-5 days after use.

6-Monoacetylmorphine (6-MAM – Heroin and Morphine), 5 µg/L

Heroin and morphine can be detected in the saliva for as much as 1-2 days after use.

Generally prescription drugs

“Road safety” limits

  • Clonazepam, 50 µg/L
  • Diazepam, 550 µg/L
  • Flunitrazepam, 300 µg/L
  • Lorazepam, 100 µg/L
  • Methadone, 500 µg/L
  • Morphine, 80 µg/L
  • Oxazepam, 300 µg/L
  • Temazepam, 1000 µg/L

With the illicit drugs, it’s easier to provide estimates on safe windows of usage, because the tests are looking for concentrations barely above trace levels. Unfortunately it’s much harder to provide an estimate of how much Valium or Temazepam will put you over the limit, because body physiology varies so greatly. We can only recommend users of these drugs consult with their doctors about what might constitute safe usage. And if you’re feeling any kind of effect from one of these drugs, stay off the road.

"These are very, very low limits," said Geoff Munro, National Policy Manager for the Australian Drug Foundation. "Take cocaine for example - 10 micrograms is absolutely tiny. Clearly some sort of educational campaign is essential. And there's a wider social consequence to talk about when you consider the limits for prescription drugs. Some people are probably going to have to give up driving, or stop taking their medication to stay on the road."

Perhaps the biggest trouble with any educational campaign will be that physiology differs so much person-to-person that simple rules of thumb will be almost impossible to make. The one drug most people know the most about – alcohol – is still extremely poorly understood by the general public, as our experiments with a personal breathalyser showed several years ago.

The only real way to make sure you're not running foul of the rules will be to self-test, an approach now endorsed by France, where every car now has to carry a personal breathalyser. But since Drugalyser machines cost over UK£2000, plus around UK£10 per test, and they take around 10-12 minutes, that's not going to be possible in this case.

The new testing procedures are expected to roll out later this year in England and Wales. Scotland is expected to bring forward its own drug driving legislation.

Thanks to drug testing experts Dr. Peter Lewis and Ashley Gurney. Also Darren Brien, Managing Director of the Drug Testing Institute and Geoff Monro, National Policy Manager of the Australian Drug Foundation.