If you have any ferromagnetic metals inside you, most commonly due to cheaply made prison tattoos, then you may feel some slight discomfort. Ferromagnetic metals are iron, cobalt, and nickel, yet the amount of iron in your blood is negligible. If you have any bullets, shrapnel, or artificial limbs inside you then that's probably the reason why you're in a hospital in the first place and someone should have told you whether or not you are MRI-able. Just follow the doctor's instructions. The average MRI machine has a magnetic field strength somewhere between 1.5 and 3 Tesla. To put that into perspective, it takes 16 Tesla to levitate a frog and around 100,000,000 Tesla to distort the electron clouds in the constituent atoms that make up your body thereby altering the fundamental interactions that occur between chemical bonds resulting in the destruction all of the biological molecules in your body and a most likely painful death as you are microscopically ripped apart. If that sounds unpleasant to you then stay away from neutron stars. Studies at both Princeton and MIT have shown that electromagnetic fields can be used to temporarily impair our brain's ability to make ethical decisions but they didn't use an MRI machine to do so. There are no studies that I know of concerning the effects of long term exposure to magnetic fields so don't sleep in a running MRI machine and you should be fine. However, I am not a doctor and none of this constitutes as medical advice.
Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to investigate the anatomy and physiology of the body in both health and disease. MRI scanners use strong magnetic fields and radio waves to form images of the body. The technique is widely used in hospitals for medical diagnosis, staging of disease and for follow-up without exposure to ionizing radiation. More: en.wikipedia.org.