A review of 10 studies of prescription drug monitoring programs (PDMP) found no consensus on how effective they are in reducing opioid use, CNN reports. Three studies reported a decrease in overdose deaths, one reported an increase, and six reported no change.

Three of the six studies that looked at heroin overdose rates found a large rise in heroin-related overdoses after PDMPS were implemented. The researchers also analyzed three studies that examined non-fatal opioid overdoses, and found one showed a decrease in overdoses after PDMPs were implemented, while two showed no change.

The programs provide doctors with access to a statewide database that includes patients’ previous opioid prescriptions, including those from other doctors or hospitals, CNN explains. This allows doctors to find patients who may be “doctor shopping” to fill multiple opioid prescriptions.

The findings are published in the Annals of Internal Medicine.