A new government study suggests some opioid-related deaths may not be counted when people die from pneumonia or other infectious diseases that are worsened by drug use.

In these cases, the death certificate may only list the infection as the cause of death, according to the researchers from the Centers for Disease Control and Prevention.

“Opioids at therapeutic or higher than therapeutic levels can impact our immune system, actually make your immune system less effective at fighting off illness,” lead researcher Victoria Hall told HealthDay. She added that the sedative effect of opioids also affects a person’s respiratory system, causing breathing to become slow and shallow. This makes a person less prone to cough, which allows pneumonia to develop.