@charm58
Effexor (venlafaxine) works by increasing and regulating the levels of two different neurotransmitters in the brain--norepinephrine (noradrenaline) and serotonin.

Serotonin syndrome occurs when you take medications that cause high levels of serotonin to accumulate in your body. Too much serotonin causes symptoms that can range from mild (agitation, increased heart rate and blood pressure, sweating and shivering) to severe (high fever, seizures and passing out). Some people are more susceptible to the drugs and supplements that cause serotonin syndrome than others, but SS can happen to anyone. It occurs most often when you combine certain medications; a number of over-the-counter and prescription drugs are associated with serotonin syndrome, especially antidepressants.

If you start a new drug, or increase the dose of a drug you're already taking and suspect you have serotonin syndrome, you should call your doctor right away, or go to the emergency room. Severe serotonin syndrome can be fatal if not treated. Once serotonin levels are back to normal, the effects go away.