MRI magnets primarily interact with the nuclear spin of protons, which can align with or against the field. Adjusting the orientation of the spin should not change any reaction chemistry, so there is no significant hazard. The nuclear spin orientations shift back into their normal equilibrium once the magnetic field is removed.

The radio waves generated for the technique can cause heating when the energy is absorbed. This should be limited by how the MRI is designed, however I am not familiar with the detailed design of MRI instruments.

More research could be done in this area for very long term effects, but I haven't seen any convincing evidence for concern.