system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is chemo more effective against aggressive cancers?

I’ve been told this by a couple of doctors but didn’t think of asking why at the time. Why is chemo more effective against aggressive cancers than slow-growing cancers?
gpt
Many traditional chemotherapy drugs work by breaking some part of the DNA replication process. This has the advantage of only hurting a cell when it tries to copy its DNA, so cells which never do that aren't affected at all. Nerves, for instance, don't copy themselves, and are therefore immune to these chemo drugs.

However, the most aggressive cancers are literally always copying their DNA. As a result, breaking that process will break a greater percentage of their cells than any others in the body, killing the cancer faster than you. This is also why chemo is bad for sperm and white blood cell count. Both are rapidly replicating cells, so both will be heavily impacted.