Pristiq, an snri, targets serotonin as the ssris do but also targets norepinephrine,
which is basically adrenalin,
which is why these aren't usually first choices for anxiety. If it's working, that's great. All of these drugs come with many side effects -- some suffer more, some less, but all drugs have side effects. They are very powerful and alter the way the body functions naturally, so naturally they throw some things out of balance. How much depends on the person.