Drug Driving & Cannabis

Get free legal advice today

Or request a call back

Drug Driving & Cannabis 


Drug Legal Limit (Blood)
Delta – 9 -Tetrahydrocannibinol (Cannabis) 2µg/L

Cannabis is a high potency plant that contains a chemical compound known as tetrahydrocannabinol (THC). It is one of the oldest psychoactive substances used by man. When you smoke or ingest cannabis, THC travels into the bloodstream and eventually binds to cannabinoid receptors throughout your body. Smoking or ingesting too much THC in a short period of time can intensify and alter its effects.

Evidence used by the government’s Expert Panel on Drug Driving has indicated that actual impairment after ingesting THC subsides after two and a half hours. The roadside test is likely to catch out cannabis smokers up to 24 hours after use, well after the effects have worn off. In fact, a study conducted by Dr E. J. Cone showed that a single ‘puff’ on a cannabis joint produced an average immediate blood THC Concentration of 18 micrograms (9 times the legal limit).

When the Government considered setting prescribed limits, they instructed a panel of experts to suggest an appropriate limit. The Government stressed its intention for the new limit to be ‘zero tolerance’. After many months of research, the experts advised a limit of 5 micrograms. Over twice the limit finally imposed by the Government.

How long before I can drive?

An average sized joint could cause a peak concentration of 120 micrograms in just 8 minutes. The legal limit is 2 micrograms.

Any given drug’s presence in your system is often measured by its ‘half-life’. This is the period of time required for the amount of drug in your system to be reduced by one-half.

Most drugs have a relatively straightforward ‘half-life’, making it simple to determine elimination rates. Nearly 100 metabolites have been identified for THC, all of which have different ‘half-lifes’. The true elimination half-life of THC is difficult to calculate. The speed at which THC leaves your system will depend on the following factors;

  • The strength and type of cannabis
  • Your height and weight
  • Your metabolic rate
  • Any medication you take

Smoking cannabis is the quickest way to absorb the THC. THC will reach a peak concentration within 3 – 10 minutes after the onset of smoking. A single cannabis cigarette containing about 25 milligrams of THC will cause an average peak concentration of 84.3 – 162 micrograms (the legal limit being 2 micrograms).

How long does a cannabis joint stay in your system?

As above, a single cannabis cigarette will cause an average peak concentration of 120 micrograms within 8 minutes. This will then drop rapidly over the following 4 – 6 hours, with a blood concentration of 2 micrograms expected to be reached within 7 – 12 hours.*

Please note that there are no ‘accepted standards’ for measuring the elimination rate of THC. The above information is based upon multiple studies and scientific findings.

Cannabis Defences

The prescribed limits for drug driving are 400,000 times lower than that for drink driving. This is because drugs exist within our bodies in minute quantities. This presents practical problems for laboratories when trying to produce accurate results. We do not intend to ‘spill our trade secrets’ on this page, but we’re quite happy to outline the four key aspects of a cannabis drug driving defence.

  • Method of Analysis

    The most common method of analysis for THC is Gas-Chromotography Mass-Spectometry (GC-MS). This is considered the ‘gold standard’ in commercial drug testing. GC-MS is a specialised analytical method that, until recently, wasn’t offered by many laboratories due to its complex (and expensive) ‘two-stage’ analytical process. It enables scientists to identify and quantify individual metabolites within a blood sample.

    Despite this method being one of the most accurate ‘large scale’ analytical methods, the laboratory are still required to deduct 30% from the measured result to allow for ‘normal analytical variation’. If this method of analysis is as accurate as the experts say, there would be no requirement to reduce the result by one third. This indicates the potential for unreliable results.  If the laboratory has failed to deduct this amount, you could be wrongly (and unfairly) charged with drug driving.

  • Quality Control and Quality Assurance Procedures

    Laboratories wishing to undertake drug testing of this type are required to attain a specific accreditation by the United Kingdom Accreditation Service (UKAS). Part of this validation process requires laboratories to frequently measure their analytical performance in terms of accuracy and precision (due to the huge margin for error with drug analysis).

    M.A.J. Law has recently received information that a leading UK laboratory has failed a number of quality control and assurance tests. It has been confirmed that a large amount of results were incorrectly calculated due to a ‘data anomaly’. The same laboratory has since had its accreditation withdrawn.

  • Calibration & Standard Deviation

    Laboratories are required to calibrate testing instruments at the beginning of each day. This process involves analysing a number of solutions (calibrants) each containing a different known drug concentration. The results of the calibration test are then plotted to produce a calibration graph. These results must fall within a validated range (the ‘standard deviation’) recommended by the United Kingdom Accreditation Service (UKAS).

  • False Positives

    No analytical method is 100% accurate. False positives do occur.

    Positive results may be obtained by consumption of non-psychoactive substances, such as Hemp Seed Oil and some forms of Vitamin B. Some illnesses can also cause reactions in the body that may produce similar metabolites to THC. M.A.J. Law has found the most common cause of false possible results to arise out of human error. Labelling errors, inadequate training, staff fatigue and boredom are also capable of producing a false positive result.

Passive Smoking – Cannabis

Secondhand exposure to cannabis smoke can produce positive drug tests. A variety of studies over recent years have documented the extent of secondhand cannabis smoke exposure and the likelihood of producing positive drug tests. A study conducted by Edward J. Cone found that blood concentration levels of THC could rise to 7 micrograms even after a short period of exposure to secondhand cannabis in a non-ventilated environment (the legal limit being 2 micrograms). These results could increase if the person inhaling the passive smoke was a ‘non-smoker’ of cannabis.

It is important to note that passive-smoking of cannabis is not a defence. It could only amount to a ‘special reason’. This is because cannabis would be present in your system above the prescribed limit of 2 micrograms (you’re therefore ‘guilty’ of driving whilst over the prescribed limit – assuming no other defences apply). If argued successfully, it could result in you avoiding a ban. If you would like more information about passive-smoking in excess specified drug cases, please call to speak to a member of our team.