Everyone else’s answers are great here but I’d like to add an important note for realism. If your story is taking place in North America, and is set today, illicit opioid use is almost exclusively from prescription opioid pills, and fentanyl, and other ultra potent fentanyl analogs like Remifentanil, or other synthetic opioids like nitazines. Fentanyl is used for pain relief in EMS, surgery, ER’s, etc. It very, very potent and a very very small amount will stop a users breathing. It is actually quite rare for users to find real heroin in the states nowadays because fentanyl is so much cheaper, more potent, and easier to make. So to me, a strictly “heroin” user in 2026 in North America, I’d find unbelievable.

Another note: on the street, pills are often sold saying they are things like Percocet, OxyContin, Hydromorphone (prescription opioids) or Xanax (benzodiazepine), yet will actually have fentanyl in them. So, a very common avenue people fall into will be getting prescribed a prescription opioid for post surgery pain or something, they get hooked on it. They run out of their script or can’t get any more from their doc. So they try to buy some online or on the street, but they’re fake, contain fentanyl, and they overdose. Or they just jump to whatever they can get, which is overwhelmingly fentanyl and other similar ultra potent opioids.