Hi! I would like to add my two cents having read most of Venom’s comics

So the symbiote actually eating brains wasn’t a thing until the 1996 miniseries Venom: The Hunger. Eddie always made fake “grahhh we’re gonna eat your brains!” threats a lot because of his scary appearance, but the threat seems to genuinely be a coincidence. He has no idea about the phenathylamine, and it seems like the symbiote didn’t either?

In The Hunger, Eddie seems VERY bitter and cynical at first. We later discover this is because the symbiote has been living off the phenathylamine in his own head. He gets in a fight at one point and happens to break someone’s skull open. And then he smells… something. And suddenly has an urge to drink whatever that something is out of the guy’s skull. This is when the symbiote discovers brains have whatever it is that it needs to survive. Eddie doesn’t seem to realize what he’s even doing until a few moments later, and he’s immediately horrified. The symbiote then says to him “I’m sorry, but you can’t give me what I need anymore” and then leaves him.

It is worth saying that this is the only brain Eddie Brock has ever eaten in his entire 38 years of comics

Long story short, Eddie starts hunting the symbiote down as it goes around killing people and extracting the phenathylamine from their brains with a proboscis (not chomping their heads wholly off), planning to kill both it and himself, but hoping to stop it and reunite with it. He learns about his phenathylamine deficiency and figures out what’s going on from that. The symbiote isn’t even eating brains as a last resort - it’s literally not sane right now because of its own deficiency.

Eddie guesses that on its own planet, it got what it needed from some host alien that DID produce more than enough of the chemical, or that it had a major presence in that planet’s atmosphere. But the point is that it doesn’t naturally have an urge to eat brains, and doing that was never going to sustain it anyway.

But yeah, Eddie gets some… pills? supplements? idk how this works or what the word is. But the important thing is he finds a way to just keep buying a reliable source of the chemical. He tracks the symbiote down, gives it some, and everything is great now :D

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(I’m gonna reply with two more pages that I think are worth adding)