[thaumasiotes]

> All I can find is that it promotes tumor growth if the tumor is already there but doesn't cause them by itself.

What's the carcinogen in chewing tobacco?

Also, the idea "it's only harmful if cancer is already present" can be weak; consider prostate cancer, which is present in all men.

[Semaphor]

Not to forget that you don't always know you have cancer. So you might cut into your time where it's still easily treatable.