Mr. Webb’s death was one of nearly 108,000 drug fatalities in the United States last year, a record, according to preliminary numbers released this month by the Centers for Disease Control and Prevention. Law enforcement authorities say an alarming portion of them unfolded the same way as his: from counterfeit pills tainted with fentanyl that teenagers and young adults bought over social media.

“Social media is almost exclusively the way they get the pills,” said Morgan Gire, district attorney for Placer County, Calif., where 40 people died from fentanyl poisoning last year. He has filed murder charges against a 20-year-old man accused of being Mr. Webb’s dealer, who pleaded not guilty. “About 90 percent of the pills that you’re buying from a dealer on social media now are fentanyl,” Mr. Gire said.

The phenomenon has led to disturbing new statistics:

Overdoses are now the leading cause of preventable death among people ages 18 to 45, ahead of suicide, traffic accidents and gun violence, according to federal data.