The problem is the similarities of cancer to normal cells. We have penicilin that works against all human cells. We call that poison.

Now, "no, i mean poisons that attack the special chemistry of cancer," oh yes, those we call chemo.

> We have penicilin that works against all human cells.

Penicillin works against bacteria, in particular gram-positive bacteria; to a lesser extent gram-negative bacteria too (this depends on the cell membrane structure of bacteria; there are other penicillin derivatives that are also more effective on gram-negative bacteria than penicillin is, but by and large the main target will be gram-positive bacteria). It does not work against human cells. If your comparison is about drugs in general, then of course cytotoxic drugs will have an effect; simplest example I can remember off-hand is colchicin. Of course it should work against cancer cells and non-cancer cells, unless there are some mutations where colchicin could no longer bind to, but that seems very very rare, due to the natural target of colchicin involved in cellular division.

For chemo it's often "these chemicals kills cancer cells faster than they kill regular cells".