The analgesic effects of fentanyl are related to the blood level of the active substance; in opioid-naive patients , minimum effective analgesic serum concentrations of fentanyl range from 0 . 3-1 . 2 ng/ ml , while blood levels of 10-20 ng/ ml produce surgical anaesthesia and profound respiratory depression .
The above sentence is very complicated. Please provide me a simplified synonymous version consisting of multiple sentences:
The analgesic effects of fentanyl are related to the plasma concentrations of the active substance . In subjects who have never received opioid therapy , the minimum blood levels of fentanyl producing an effective analgesic effect are between 0 . 3 and 1 . 2 ng / ml .