I would like to add to the answer by Bigjoemonger that isotopes of stable nuclei may be radioactive as seen in this chart.
Take carbon, as an example, which is in all organic matter. It has two stable and one decaying in 20 minutes with a β+. Its electronic composition changes and it is a Boron isotope which will replace it in the lattice of your material. This is stable but the chemical composition is no longer appropriate for organic matter ,and there will be radiation damage from the e+.
Take iron, as another example.There are three stable ones, 56Fe, 57Fe and 58Fe, and two unstable. You could choose between the three stable ones and there would be only the effects described in the other answer.
In general in the substitutions you want check carefully the isotope stability content, a radioactive one will 1) introduce radiation damage and danger 2) after decay the nucleus will be a different one with different electromagnetic properties and the chemical bonds will be broken.