But meth, it turns out, was only on hiatus. When the ingredients became difficult to come by in the United States, Mexican drug cartels stepped in. Now fighting meth often means seizing large quantities of ready-made product in highway stops.

The cartels have inundated the market with so much pure, low-cost meth that dealers have more of it than they know what to do with. Under pressure from traffickers to unload large quantities, law enforcement officials say, dealers are even offering meth to customers on credit. In Portland, the drug has made inroads in black neighborhoods, something experienced narcotics investigators say was unheard-of five years ago.

“I have been involved with meth for the last 25 years. A wholesale plummet of price per pound, combined with a huge increase of purity, tells me they have perfected the production or manufacturing of methamphetamine,” said Steven Bell, a spokesman for the Drug Enforcement Administration. “They have figured out the chemical reactions to get the best bang for their bucks.”

Nearly 100 percent pure and about $5 a hit, the new meth is all the more difficult for users to resist. “We’re seeing a lot of longtime addicts who used crack cocaine switch to meth,” said Branden Combs, a Portland officer assigned to the street crimes unit. “You ask them about it, and they’ll say: ‘Hey, it’s half the price, and it’s good quality.’”

Nationally, nearly 6,000 people died from stimulant use — mostly meth — in 2015, a 255 percent increase from 2005, according to the Centers for Disease Control and Prevention. The percentage of the nation’s drug overdose toll that was attributed to stimulants inched up to 11 percent of the deaths.

United States Customs and Border Protection statistics show that in the past five years, the amount of meth seized has tripled, while the seizures for other drugs have declined or had only modest increases.