Critics of the global War on Drugs regularly point out that narcotics tend to be more dangerous when they are illegal, as their production and sale is not subject to any regulation or quality control. This means that people often have no idea what they are really getting when they buy drugs from a street dealer. New data shows that a quarter of all drugs purchased in the UK contain substances other than those advertised.

Due to be published this week in the British Journal of Clinical Pharmacology, the new study breaks down findings obtained by a non-profit organization called The Loop, which provides drug-checking at festivals and nightclubs so that users can see what’s in their drugs before deciding whether to take them.

Tests were carried out over five days in Bristol and Durham, which the study authors describe as “contrasting cities” due to their demographic and economic profiles. A total of 171 drug tests were carried out, with classic recreational drugs like MDMA, cocaine, and ketamine making up 69 percent of these.

Results showed that 24 percent of drugs handed in for testing were not what they were supposed to be, with some MDMA samples being found to contain anti-malaria medications, while some cocaine samples consisted of baking soda and caffeine.

A number of highly dangerous adulterants were also found in many drugs, with substances like pentylone sold as MDMA. While pentylone does produce effects that are similar to those of MDMA, it tends to last for a much shorter time, meaning people end up taking more of it. Yet redosing also increases the amount of time that it takes for pentylone to clear the body, resulting in a range of unwanted side effects such as insomnia.

Drug users were given the opportunity to speak to health professionals for advice after receiving the results of their tests, with just under 30 percent of those involved in the study deciding to lower their dosage, while roughly one in 10 chose to dispose of their drugs altogether.

In addition, the discovery of dangerous adulterants allowed organizers to spread messages on social media warning people about contaminated batches of drugs.

The Loop first began offering drug tests at festivals in the UK in 2016, resulting in a 95 percent decrease in drug-related deaths compared to the previous year at these events. However, opponents of the scheme have argued that this service simply encourages drug use, which is why it hasn’t been widely adopted.

Yet with drug-related deaths currently at their highest ever levels in the UK, harm reductionists are now calling for more drug checking facilities at festivals and nightclubs.

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