The drug, which is legal for medical use in the majority of US states, is mainly prescribed to ease pain. But this new research conducted in Colorado - which was the first state to legalize - suggests that short-term pain relief could weaken the body's resilience to pain over time. The researchers, from the Swedish Medical Center, Colorado, analyzed around 260 people who were involved in minor vehicle accidents and admitted to trauma centers. Of these, 54 tested positive for recent marijuana use while 16 claimed they used the drug more or less every day.