Drug overdose deaths are decreasing sharply across the country, according to recent state and federal data, a dramatic improvement in the nation’s efforts to reverse the consequences of fentanyl’s spread in the illicit drug supply.

Between April 2023 and April 2024, overdose deaths declined by about 10 percent nationally to roughly 101,000, according to preliminary data published recently by the Centers for Disease Control and Prevention. That amounted to the largest decrease on record, according to the Biden administration. Nonfatal overdoses are also down more than 10 percent.

The data suggests that some of the tools used to combat opioid overdoses, such as naloxone, the overdose-reversing medication, were having a significant impact. But researchers and federal and state health officials have puzzled over the exact reasons for the decrease, including why overdoses have fallen so much in recent months.

The pace of the decline “is such an anomaly in the last 20 years,” said Nabarun Dasgupta, a leading drug policy expert at the University of North Carolina at Chapel Hill who published an analysis this week of the state and federal data.

Some states have reported even greater decreases than the national rate. In Kentucky, overdose deaths dropped by more than a third between April 2023 and March 2024. Arizona, Maine and Vermont all recorded recent decreases of around 15 percent.

North Carolina’s fentanyl overdose rate fell by more than 30 percent from May 2023 to May 2024, Dr. Dasgupta said, a figure that prompted him to call the state’s health department to confirm that the number was real.