Opium literally gets scraped off of poppies. Anyone can actually obtain it with some patience and know how. It would take an enormous individual effort to collect enough to use, let alone hurt oneself or sell any.

Yet, ridiculously, sometimes people who have simply eaten a lot of poppy seed bagels or something end up getting tagged as heroin users, because the metabolites are so similar. Never mind that the quantity of poppy seeds you'd have to ingest in order to feel even a little bit of opiate effect is probably more than a human can physically consume. It gets picked up and flagged because the tests are that sensitive.