Firm confidence in G‑d can perform miracles of healing. Nevertheless, you should still follow carefully the instructions of your doctor.

Not that it is the doctor or his medicine that heals
—it is the Healer of All Flesh who heals.

But the doctor and the medicine provide a natural channel for the healing to occur. Such is the way G‑d prefers His miracles to work—through the natural means that He has created.