@ Mia: I am going to go back to your original question about Pristiq.

Pristiq is an improved version of the older drug Effexor. It is the active metabolite of Effexor and is thought to work better and more smoothly since the 1/2 life is longer in the body... 11 hrs compared to 4.5 hrs for Effexor.

Here is the Wiki on it:
http://en.wikipedia.org/wiki/Desvenlafaxine
You can see there is some discussion on whether or not it works well in some patients (depending on race and metabolic genetic systems).

This is the wiki on Effexor:
http://en.wikipedia.org/wiki/Venlafaxine
There is more information on this because it has been used much longer than Pristiq.

People will vary in their response to these drugs because everyone is not uniform with neurotransmitter status. Some people have more serotonin than others, and the same goes with other neurotransmitters. So one person may have serotonin excess and side effects, and another would not.
The same also goes with libido effects, as well.

Also with time, drugs that affect neurotransmitters may cause the brain to change in some ways with their presence. Some doctors have observed lowered dopamine with time when SSRI or SNRIs are used. And some people may be more likely to have this effect as well compared to others.

If you have any other specific concerns, Mia, you can PM me anytime. I am always here.