I am not a psychiatrist, but mental health counselor with over 20 years of experience in dealing with medication issues. Both ssri's and anri's are are anti depressant with an anti anxiety component. The ssri's deal with the seratononin system, a neurotransmitter, a snri deals with seratonin and the norepinephrine system. The ssri's deal exclusively with serotoninThe lack of these neuro transmitters is thought to cause problems with depressed mood..Effexor, Celexa and and Cymbalta are examples of srni's where as prozac is an example of a ssri. Example of major ssri side effects are: Insomnia, weight gain, sexual dysfunction Some reports indicate fewer side effects with srni's especially sexual disfunction.are nausea constipation and weight loss. But each individual is different and may may or may not have side effects to one degree or another. That is why you are started out slowly and built uo to a therapeutic dose (titrate). Iff you found this answer useful please presss the reen button so I know and can be be paid. Feedback is always appreciated.
Doctors get comfortable with the results of a particular drug and tend to stay with it. Maybe they have researched it further, etc, The other reason is that some drugs have less side effects, some are sedatinmg while others are energizing. It really is trial and error based upon experience and particulars of the depression or anxiety etc as to why one drug is chosen over another.