The book Fallen Glory: The Lives and Deaths of History’s Greatest Buildings by James Crawford has a chapter about the walled city of Kowloon in Hong Kong, and there is also an article based on this published at Atlas Obscura.
The claim I'm doubting appears in both publications:
Many of the city’s rats were addicts too, and could be seen writhing in torment in dark corners, desperate for a hit.
A footnote points to another book as the source of this, City of Darkness by Greg Girard and Ian Lambor. In an interview on p. 169, a former resident of Kowloon named Peter Chan indeed states:
Rats would come around as well. They were addicted too, and they twisted and turned, longing for a fix.
Is there any further evidence to support this statement as more than urban legend? Are there other eyewitness accounts? Does laboratory science support the possibility that rats would actively seek out opiates or other drugs in the environment of their own accord, and that the observed behavior could be a sign of withdrawal? I'm assuming that people would not intentionally feed opium or heroin to rats, but any evidence that this occurred would also be relevant.