A new study looked at how a patient’s first opioid painkiller prescription affects their chances of becoming a long-term opioid user.

A review of the patient records of about 1.3 million people from 2006 to 2015 followed patients with no history of opioid abuse from their first opioid prescription.

The study’s findings weren’t really a surprise—the longer the duration of a patient’s first opioid prescription, the higher the likelihood that they would still be taking opioids a year later. 

But the researchers, led by Bradley Martin of the University of Arkansas for Medical Science, were able to quantify the transition to chronic opioid use. 

The study found that the probability of becoming a long-term opioid user is established early, determined by a number of factors including how long the initial prescription lasts, and whether a patient gets a refill.

Those who received just one day’s supply of painkillers with their first opioid prescription had a 6% chance of continuing opioid therapy for a year or longer. 

Though Martin says first opioid prescriptions that give a 30-day supply are rare, they can increase one’s chances of relying on painkillers for at least a year by 45%.

The researchers noted that just 7% of the study sample received longer term prescriptions—most patients received a prescription for one week’s worth of painkillers at a time.

And among those who received a refill, 1 in 7 were still taking opioids a year later. 

Martin and his colleagues hope their analysis will help doctors make smarter choices when prescribing opioid painkillers. The overprescribing of the powerful drugs has come under scrutiny as the overdose death toll has steadily risen over the last decade.

The CDC reported that overdose deaths involving prescription opioids have quadrupled since 1999. Since then, more than 183,000 people have died from overdoses related to prescription opioids. 

Last year the CDC issued new guidelines for doctors prescribing opioids in response to the growing epidemic of heroin and prescription painkiller addiction in the United States.

The guidelines—which focus on treating chronic pain and not cancer, palliative care or end-of-life care—emphasize that physicians should try to prescribe non-opioid pain medication whenever possible. And if a patient does require opioids, the CDC recommends starting them on the lowest possible dose.