Eli5 – Why do some cancer medications cause cancer?

Question

I take a medication (Methotrexate) to treat rheumatoid arthritis. The drug is also used to treat cancer in higher doses. Certain types of cancers are a potential side effect. How does a medication used to treat cancer also potentially cause it?

In: 7

Answer 1

The basic idea of treating cancer is that cancer cells are weaker than normal cells. So you try to damage cells, aiming for the specific area with cancer, just enough so that it kills cancer, but it does not kill many normal cells. The reality is that you are gonna have some normal cells hit by whatever method you use, and that can itself cause cancer.