In TLD John asks Sherlock if he needs another hit, Sherlock says it can wait until the hospital, and John just LETS HIM??
I KNOW Lovely. I was SO CONFUSED. John is SO OOC in TLD. One of the many reasons why I think S4 is someone’s dream or “Unreliable Narration”. That, or John is so depressed into his own head, he just doesn’t care anymore. We can see this at the end of the episode that he was just bottling up a lot of his feelings.
Another thought is that John knows of the plan and is begrudgingly going along with it, but I doubt it. Given that Sherlock himself knows John would not approve of the drug use (as seen in HLV and TAB), then I don’t think it’s likely John would allow Sherlock to just keep taking drugs.
Another theory is that Eurus is planting improper memories into John, which is why he attacked Sherlock and why he’s so blasé with Sherlock’s drug use.
Regardless of what the theory is, our John would NEVER be okay with Sherlock just taking drugs all willy-nilly. So many things with S4 sat wrong with all of us, and John’s improper character is one of those “wrong” things.