  Over the last few decades, a plethora of modifications to general relativity
have been proposed to solve a host of cosmological and astrophysical problems.
Many modified gravity models are now ruled out with further astrophysical
observations; some theories are still viable, with, at best, bounds on their
parameters set by observations to date. More recently, observations of Fast
Radio Bursts have proven to be remarkably powerful tools to constrain cosmology
and fundamental physics. In this work, we consider a generic modified gravity
theory and consider the implications for gravitational lensing with Fast Radio
Bursts. We use a set of Fast Radio Burst observations to constrain the fraction
of dark matter made up of primordial black holes in such a theory. We further
show that modified gravity adds a screening effect on gravitational lensing
similar to the case when there is plasma in the path of the light ray acting as
a scattering screen.
