An Effexor overdose is possible, if you are not careful. The elderly are even more susceptible, because their bodies metabolize the medication more slowly. So, the drug tends to build up in the body over time. Actually, it is a wonder that this medicine is still on the market. It is meant to treat depression and anxiety disorders. But, it also increases suicidal thoughts and behaviors. In fact, it now has a black box label to warn uses of the potentially deadly side effects. If the patient is also taking medication for migraine headaches, he/she can develop Serotonin Syndrome, which can be fatal. If you experience any negative side effects, it is extremely important to contact your doctor immediately.