That's not really a harm reduction questions, so this thread might get closed
It doesn't have to be harm-reduction in this section as far as I'm aware....this is Drug Culture. I'm pretty sure this area is for chatting, discussion, and even silly stuff about drug use and the way users are impacted, not only harm-reduction topics. I don't think the "How high am I" and "I Shit in My Neighbor's Pool Last Night" threads I've seen here are harm reduction....just talk about drugs and use in general. Plus, this is a valid question and the answers teach about drugs, addicts, and the harm that can come from them. That is harm-reduction as well. Sorry, not trying to upset anyone, I just don't want new members to think that they can't express themselves and ask questions if they don't directly pertain to HR. You're fine posting questions like that here IMO. Mods....please correct me if I'm wrong!
Now....to answer your question......
It is a very common progression of addiction for that to happen. If someone abuses a milder drug like Codeine or Vicoden, most of the time, they will eventually advance to stronger drugs. Not always, but most of the time. There are a few reasons why this happens.
One reason is money. Some of the stronger drugs (heroin for example) is rather inexpensive when someone first starts using it. That's because their tolerance is low enough that they do not need much at all to achieve the high they got from much larger, more expensive doses of other opies. It very quickly becomes more expensive because heroin tolerance rises quickly, but that is beside the point.
Another reason is the composition of the drug itself. Lower potency drugs typically contain large amounts of other medication with the opiate/oid. Codiene and Vicoden for example have Acetominophen. Percocet has Ibuprofen. As one's tolerance increases, the need for more medication rises in order to feel the same effects, and they reach a point when so much is required that the APAP content is too high to be safe. That's when the user graduates to stronger drugs, in order to eliminate the APAP problem. Oxycodone and other products do not conain APAP, so they can take the higher doses without the danger.
For some, it's purely addiction that causes the change. They just simply cannot achieve a high anymore from the weaker opies, so they "upgrade" to stronger ones. It's just the evolutionary process of addiction. Just like when you first started drinking, it took one beer to get you drunk. Then, over time, it took two, then four, then twelve, then whiskey.
Access also can sometimes play a role. Perhaps their dealer moved or went to jail, they lost their prescription, or their doctor retired. They cannot access their drug of choice but need something to keep the withdrawals at bay, and to get the high they are after, so they find a replacement.
Hey, maybe someone will come across this, read it, and be turned off drugs before becoming addicted....if only it worked that way :/.....
There, now it's harm-reduction

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