Is Personalized Medicine Enabled in Clinical Trials?

I've heard it said that personalized medicine enables more efficient and effective clinical trials. And I'm wondering if you couldn't help me understand why that's true. So what happens is some research are like me in the lab, identifies the molecular mechanism a cellular pathway that when needs to prepare causes the disease. Second step is to find the chemical compounds or the chemical compounds that can actually stop this aberrant pathway from occurring. This is the drug then having identified the drug that affects the specific molecular mechanism. We're going into clinical trials. But of course potential problems with that, because as I said earlier, clinical trials also test for potential adverse effects of the drug.