Fluoride is very common in synthetic drugs but must be present in non-reactive groups as p-fluorophenyl or trifluoromethyl.
The radius of the van der Waals of fluorine is only about 20% greater than the radius of hydrogen, therefore enzymes and receptors do not notice a difference in size, but enzymes are unable to metabolize such a group, e.g. oxidize the phenolic group, especially in the para position or to a carboxyl group.
In addition, fluoride has a beneficial effect when drugs overcome lipid barriers, i.e. the drug gets to places where it would not reach due to the rapid metabolism of the unsubstituted compound.