Are you talking about this page?
https://en.wikipedia.org/wiki/Opioid_epidemic
Could you then be more clear where exactly your claim came from? I did not find it, but rather this:
"According to medical professionals, supervised injection sites are effective in reducing overdose deaths and the transmission of infectious diseases."
"From 1999 to 2020, nearly 841,000 people died from drug overdoses,[7] with prescription and illicit opioids responsible for 500,000 of those deaths"
Here's a chart showing overdose deaths from all drugs in the US- yes there's definitely a large spike from 'synthetic opioids' at the end there that's probably all illegal fentanyl. But notice the blue line for 'prescription drugs' was very very steady for the entire length of the chart. That's an enormous number of deaths from completely legal, regulated drugs!
https://upload.wikimedia.org/wikipedia/commons/f/fa/US_timel...
So... it's not saying 500K deaths are from prescription drugs, it's saying 500K deaths are from opioids (some of which were prescription)
Also, in the chart you mentioned, it's not clear if the opioid deaths were from legally produced prescription opioids. Certainly fentanyl is one of the biggest killers, and fentanyl is used in medical settings, but the fentanyl killing people is usually not from legal drug manufacturers.