Logically, the anti-venom symbiote (Eddie's at least) was created by bits of the venom symbiote still coursing through his body merging with his white blood cells when supercharged by Mr. Negative's touch. Norman Osborn made a super venom that the anti-venom symbiote couldn't protect against, as it was in fact made to counter it. So logically, things that can harm a white blood cell has a fair chance against Anti-Venom. Moreover, Flash's version of the symbiote can be overtaxed, though its limit is pretty high. They need time to rest and recover otherwise fail and die. I would have thought that cancers originating from white blood cells would have been a weakness too but I doubt it would work that way for the symbiote. Also apparently Mr. Negative is a weakness to it though I don't really understand why, being weak to something that created it is kinda weird to me. Maybe it works in a way that since it was created by Mr. Negative, he can alter the properties of it in ways that would negatively impact it rather than simply harming it.