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How do the police test for drug driving and how long do they have to prosecute?

View profile for Paul Loughlin
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The consequences for a conviction for drug driving are serious. If you’re convicted, you face a minimum one-year driving ban, an unlimited fine, up to six months in prison and a criminal record. Your driving licence will also show that you have been convicted for drug driving - for 11 years.

All this can have negative implications for your employability, the costs of your car insurance and your ability to travel to certain countries.

But how can you be prosecuted for drug driving? How will the police test to prove that you have driven under the influence of drugs? And how long do they have to charge you if that test comes back as positive?

It’s important here to explain the difference between legal and illegal drugs, and the implications for driving. It’s possible for a prosecution to be brought against someone who might have driven under the influence of legal prescription drugs - not just if they have used illegal drugs. It’s also important to provide some background on the two laws which can be used to potentially bring a prosecution and secure a conviction.

In 2015, a new offence was introduced which set specific legal driving limits for a suite of 16 legal and illegal drugs. If you are found to be over the limit for the eight illegal drugs covered by the new offence, then you can be charged with drug driving.

For the eight legal (prescribed) drugs covered by the act it must be proven first that the drugs have not been prescribed by a healthcare professional - and that you were over the prescribed limit.    

In order to bring a prosecution, the police must first prove that you were driving at the time of the alleged offence. And that the proportion of a drug specified in the Act was over the prescribed legal limit.

Prior to this new offence driving while unfit through drugs was covered as a so-called ‘section 4 offence’ (section 4(1) Road Traffic Act 1988). This act and the offence associated with it - also known as driving while unfit - are still in force however they rely on the police’s ability to prove a driver’s ability to drive was impaired because they were under the influence of drugs. The Act set no specific limits for legal or illegal drugs.

In most cases however, prosecutions are now brought using the new driving with excess drugs offence. The 2015 changes were implemented by an amendment to the 1988 Act to include a new ‘section 5A offence’ (section 5A Road Traffic Act 1988).

How do the police test you for drug driving?

Under the section 4 offence mentioned above, testing was carried out at the roadside using a Field Impairment Assessment. This may still be the initial method used by police if they stop you on suspicion of drug driving.

A Field Impairment Assessment involves things such as asking you to walk in a straight line, balance on one leg or touch your nose with your finger. It is widely recognised however that this test is not wholly reliable or objective.

The police can also ask you to provide a saliva sample at the roadside and this allows them to screen for the presence of cocaine and cannabis. This, on it’s won, does not establish impairment but is an important element towards establishing the presence of drugs as being the reason for any impairment.

If the officer at the scene concludes that a driver has failed either the FIA test, or the saliva analysis has shown a positive result, they can be arrested and taken to a police station and asked to provide a blood sample. Under the section 5A offence, the results of this sample can be used to bring a prosecution if the limits prescribed in the Act have been exceeded and it is demonstrated you were driving at the time.

How long does the process take and how long do the police have to prosecute?

The results of the blood test, which are carried out by a doctor rather than a police officer, are not provided immediately at the police station. The sample provided will be sent away for analysis at a forensic science laboratory. It can take between six to eight weeks for the results to come back – sometimes longer.  

If a positive result is confirmed, a charge is likely to be brought – that of driving with excess (over the specified legal limit) drugs and the prosecuting authorities - the police and the CPS - have six months to charge you with an offence.

It’s important to seek professional legal advice if you have been arrested on suspicion of drug driving so that you have the best chance of defending your case.

Stephensons has significant experience of defending individuals facing serious motoring offences including drug driving. Our specialist solicitors are here to help should you be investigated or charged with drug driving and have a proven track record in securing acquittal. Call us today on 0161 696 6250.


    • Drink driving - breathalyzer Jay
    • Posted

    I was arrested at 11.30pm in uk for drink driving but was not breathalyzed until 04.30am 5 hours later.can the police use this reading 

    Response from Stephensons

    The level of alcohol measured in your system at the time of the breath test is presumed to be the level as of the time of the allegation of driving. This is supported by section 15 Road Traffic Offenders Act 1988. Substantial delays in obtaining a specimen after arrest may result in the police carrying out a ‘back calculation’. This would usually happen where the police believe that the reading is likely to be substantially lower because of the delay and the calculation will be to determine the level of alcohol at the time of alleged driving.

    In essence, the police can rely on a breath reading taken 5 hours later. If you would like to speak to a member of our team to discuss your situation please call us on 0161 696 6250.

    • Appeal for my son Sadie Clifford
    • Posted

    Today we had a shock of 17months ban and 340 fine , firstly it was his first offence , secondly it was 6months 8 days ago , bloods only taken on 12th April , 2022 no breathalyser working , he admitted to two lagers , got we feel a heavy sentence and there was also special reasons as has mental health and really needs his car for work 

    Response from Stephensons

    For more information about appealing a motoring offence conviction please click the following link: Appeal driving conviction

    If you would like to speak to a member of our team please call us on 0161 696 6250.