Drug checking has been shown to be an effective way to reduce the harm from drug use through informing safer use, limiting use, and helping users avoid the most dangerous substances. The services also provide monitoring and detection of new psychoactive substances to inform public health interventions. The Loop have stated that 20% of samples are handed in for disposal and 40% of service users reduce intake. KnowYourStuffNZ have found that, when substances are not as expected, half of service users state they will not take that substance and a quarter say they will take a smaller quantity. Drug checking services also reach drug users who are not reached by existing services. Evidence from research conducted by Austrian pill testing service CheckIt! found 58% of people who use the service would not otherwise seek out harm reduction information, and about 75% are more likely to access harm reduction services if pill testing is included. 
Academic research from the UK has found that one in five substances were not what they were expected to be and two-thirds of misrepresented samples were disposed of. Such on-site testing accesses otherwise hard-to-reach user groups to reduce the harms associated with drug use.
Research that followed-up people who had used drug checking services in the UK revealed that those people acted upon the harm reduction advice that they received from the service. Those people disposed of unwanted substances, reduced their dosage of wanted substances, and reduced their risk of overdose. People also followed those risk management practices after attending festivals, alterted friends to the risks of drug use, and continued to follow that advice.
In a peer-reviewed study published in Journal of Psychopharmacology, researchers at Johns Hopkins found that people were about half as likely (relative risk = 0.56) to report intent to use a product if testing did not identify the substance as MDMA, and this was a statistically significant reduction.