The alternative is thinking of the body as a chaotic system which settles into various attractors. Take the wrong drug and you can push yourself into a different attractor state, which will persist until something shifts it. This definitely seems true of some things, and is one of the ways I think about depressive episodes - which can last months or years, and which can be precipitated by some sort of obvious stressor (getting fired, breaking up, being bullied) but last long after the stressor is gone. If this explains permanent drug side effects, it seems somehow scarier to me than the other option. It's not just that you have to make sure not to accidentally kill any cells. It's more that nobody has any idea what the underlying mechanisms look like, anything can happen, and you just have to hope you don't screw up.