I'm sorry but that actually is not true about IV users. Acute withdrawals only last a few days for IV users, peaking around day 3 or 4 and lasting around a week, same as with any other method of use. The thing that would affect withdrawal severity more than anything else would be the amount he had been taking, but there is no such thing as it getting into the bones and spine and rereleasing. No matter what, those severe withdrawal symptoms will be over relatively soon. Post acute withdrawal syndrome (PAWS) last longer but those symptoms are not as physical but more mental. PAWS is the reason that a lot of people relapse, because they are not mentally ready to cope with the world and this is why Ryan NEEDS to stay in rehab and why leaving early simply does not work.

https://americanaddictioncenters.org/withdrawal-timelines-treatments/heroin/