Well um to explain it, morphine is a drug. An addicting drug, which is why there are strict limits on it, as to not overdose or get addicted. I've been on morphine, and honestly the withdrawal was worse then pain itself. I'm not sure what country you are from, but in countries like USA the drag distribution is lot less strict then in Europe or Japan. And also after they take away morphine you're still allowed to revive slightly stronger versions of pain killers like ibuprofen. Hope this explains.
Also I'm not sure how Thailand drug distribution works but from what happened in the manga it seems it's similar to Europes
Why the fuck didn't she get any pain reliever anymore!? The fuck!?
I remember after an operation, my father could use that morphine thing for as long as he wanted while recovering. He got some of his lung cut out.
That kind of pain definitely justifies its usage against the risk of addiction!