As Donald Trump travels to Beijing this week, fentanyl – and China’s role in its supply chain – remains an enduring point of acrimony in bilateral relations .

At a UN meeting in March, the US again accused China of failing to stop its chemical industry selling the precursors required to make the potent synthetic opioid , while China suggested the US was shifting the blame for its domestic drug problem.

Yet there are growing signs that the US fentanyl crisis has turned a corner – and some experts believe that interventions made in China have played a key role.

“There was a supply shock: the purity of fentanyl fell,” said Keith Humphreys, a professor at Stanford University. “The question is why was there a supply shock.…