Good thinking; I'd assumed that they weren't designed to enter the brain, so were less lethal than intended. (Not a medical doctor, nor a biologist, but I assume that it's easier to get drugs to the intestines than it is to the brain, and that 'poorly absorbed into the bloodstream' is a feature, not a bug, for anti-tapeworm drugs, since it's less likely to mess with human cells.) But that depends on how much the symbiote can rely on the human brain tissue and how much it relies on its internal processes (and for what).

But if Sal could be scared off of taking them on her own, that would be for the best.