What is the motivation behind fluorinated pharmaceuticals?

Question

From what little research I've done, it appears that a significant portion of modern psychoactive drugs are fluorinated in one way or another (for example, Buproprion, Fluvoxamine, or any number of other antidepressants).What is the motivation behind the addition of these groups? My initial guess would be that it either increases blood-brain barrier clearance or reduces the drug's metabolism, but I've been unable to find papers supporting either idea (or papers on the subject at all, for that matter). Any books/papers illuminating the subject would be greatly appreciated.

Answer 1

Broadly speaking, the structure activity relationship especially of most antidepressants suggests that fluorination/a halogen improves drug affinity to the receptors for drug ligand interaction especially a O-H or N-H group.

The position determines the potency in most cases.