Cannabis has become stronger over the last two decades, a new study has revealed.

Scientists have analysed almost 39,000 samples taken from cannabis which was seized between 1995 and 2014 by the US Drug Enforcement Administration. The study found the levels of tetrahydrocannabinol (THC) found in the drug had got higher over time.

THC is a psychoactive chemical found in cannabis which can cause hallucinations. The research showed that in 1995 the average level of THC in the impounded drugs was 4 per cent but by 2014, this had grown to 12 per cent.

The study, which was carried out by the University of Mississippi, suggested that the increase in potency meant cannabis was now more dangerous than in the past, particularly for young people.

And in the US the number of people visiting hospital emergency departments after taking cannabis has increased. Figures show that the number of cannabis users visiting accident and emergency had risen by 53.6 per cent for men and 42.9 per cent for women between 2005 and 2010.

Study finds drug is now more psychoactive

The research, which was published in the journal Biological Psychiatry, found that while THC levels had gone up, the amount of cannabidiol (CBD) had actually gone down. Unlike THC, CBD is not mind-altering and has fewer side effects.

The study found that the CBD content in the samples in 1995 was almost 0.28 per cent but by 2014 this had fallen to 0.15 per cent. And the ratio of THC to CBD had gone from 14 times in 1995 to almost 80 times in 2014, suggesting major changes in the make-up of cannabis being sold by dealers.

Cannabis is the most widely used illegal drug in the UK. Official statistics from the 2014/15 Crime Survey for England and Wales found that 6.7 per cent of people aged between 16 and 59 had used cannabis within the last year.

The high number of cannabis users is a cause for concern for many employers who are worried that drug use among staff could compromise health and safety and reduce productivity.

Random drug testing is one way for workplaces to monitor the use of cannabis and other illegal substances among their employees. AlphaBiolabs also offers post incident tests which can determine whether drug use played a part in an accident or disciplinary matter.

And some companies are now choosing to use drug tests as part of the recruitment process, particularly for roles in which safety is critical.