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?
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It’s kind of hard to say. The mechanisms of action of MTX are just not entirely understood, it’s something of a black box drug. When used at high doses as a chemotherapeutic, MTX strongly interferes with a cell’s ability to produce the building blocks of DNA/RNA, and that means trouble when that cell enters the phase of replication where it must copy its DNA but finds itself short on materials. Generally when a cell is unable to continue replication normally, it has no other option than to call it off and go into apoptosis (controlled self-destruction). This sort of approach is common to many chemotherapeutics because cancerous cells are on average spending a lot of time replicating, so they’re susceptible to something that harms them mid-replication.
Now, as some of the other comments are saying, there’s always some possibility of survivable damage in a cell that wasn’t cancerous, but becomes that way because of it. But I would speculate in a different direction, because the association with increased cancer rates is also present when MTX is taken at much, much lower doses as an immunosuppressant for conditions like rheumatoid arthritis. Low dose MTX exerts quite different effects, and doesn’t interfere so broadly with DNA building block production –> possibility of survivable DNA damage. How it works instead then is *also* not entirely understood, but it’s thought to involve a more subtle interference that keeps T- and B cell activation down. That can be desireable when those are causing an autoimmune disease like RA… but they are also responsible for detection and killing of cancerous cells before they get out of hand, particularly T cells.
So to summarize a bit, yes, MTX is used as a chemotherapeutic in cancer, but at different dosage it can do different things — namely, become more of an immunosuppressant (though since these immune cells are also rapid multipliers, high dose MTX hits them too!). And immunosuppression in general can make you more susceptible to developing various cancers, because your immune system is normally in charge of killing off cancerous cells before things get out of hand. You’ll see the same thing with other immunosuppressant drugs used to treat rheumatoid arthritis, like TNF inhibitors.
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