As others have said no one really knows why they work in some people and not others. Everyone is so different. Everyone metabolizes things much differently and that in large part is what causes such a wide range of reaction.

The SSRI's and SNRI's for example - they have a pretty good understanding of what they do in the brain as far as reuptake of neurotransmitters in the presynaptic cleft and how they effect receptors on the neuron and dentrite on both sides of the cleft. Why one would work for you and not another or why one would work for me and not you.....who knows. It is a crap shoot. I believe it has a whole lot to do with how we metabolize them and what active metabolite our liver produces. There are now a couple of genetic tests available that will tell you how you will metabolize a set of drugs but most pdocs don't have access to them yet and they may not say anything about effectiveness and only tell you about side effects. So a pdoc really has no idea how you will metabolize it or how effective it will be.

I also work in partnership with my pdoc and it is often me who throws the dart. My guess is as good as his. It is a game of try it and see what happens. What side effects you get and will they go away. Can you tolerate the side effects for awhile or the whole time you are on it. Often it takes four to six weeks to find out if it is even working.

This is not unusual. It has always been this way for regular medicine as well. Trial and error and they learn little bits at a time. A drug may work very well for something but they have no idea why. Blood pressure meds were first discovered totally by accident. They may have a good understanding today how blood pressure meds work but it took many years of study. And you would think that would be pretty straight forward and easy to study. I read once that there are 200 factors that contribute to high blood pressure. If that is true then it is pretty complicated. Compared to the brain it is very easy to study.

The brain is extremely difficult to study. You heart they have learned a lot about. They can run cameras up your arteries. They can do live very hi res color ultra sound images and all kinds of things. Your brain they can't just cut open and see what is going on.

As Sister Rags said it is as much an art as it is a science. That's why they call it the medical arts.