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Side Effects of Effexor and Weight Loss

By Elizabeth Wolfenden ; Updated July 18, 2017

Effexor, the brand name of the drug venlafaxine, is approved by the FDA to treat anxiety and depression. Although Effexor may benefit those suffering from these conditions, the medication may also cause several side effects. While most of the side effects are harmless or temporary, certain side effects may be dangerous and even life-threatening. Always discuss all potential side effects of Effexor with your doctor before taking the medication.

Effexor and Weight Loss

The most common side effect of Effexor is nausea. This side effect occurred in up to 58 percent of people in clinical trials. However, other gastrointestinal problems are also common. In clinical studies, up to 17 percent of people experienced a loss of appetite, up to 15 percent of people experienced constipation, up to 8 percent of people experienced diarrhea, up to 6.8 percent experienced vomiting and up to 6.7 percent of people experienced indigestion. These gastrointestinal side effects may trigger weight loss in some individuals. In fact, on average, people lose about 1 to 2 lb. when they begin taking Effexor, according to However, you may not experience these side effects at all when taking Effexor. In fact, you could even gain weight. If weight loss is your goal, speak with your doctor about a medication specifically intended for weight loss.

Other Side Effects

Effexor also has many other common side effects. In clinical studies, up to 25 percent of people experienced headaches, up to 23 percent of people experienced drowsiness and up to 22.5 percent of people experienced insomnia. Emotional and mental side effects also may occur. Up to 21.3 percent of people experienced nervousness and up to 11.2 percent of people experienced anxiety in clinical trials while using Effexor. Confusion, agitation and unusual thoughts occurred in 1 to 6 percent of clinical trial participants. Sexual side effects are also common. Ejaculation problems occurred in up to 12.5 percent of people in the clinical trials, impotence occurred in up to 6 percent of people and a decreased sex drive occurred in 1 to 6 percent.


Sometimes doctors prescribe medications because they believe the side effect of the medication may benefit the patient. This act is called off-label prescribing. Since Effexor may cause weight loss, some doctors may prescribe the medication specifically for that purpose. However, taking this medication solely for weight loss may be dangerous. While serious side effects of this medication are rare, they do occur and this potential risk should not be taken lightly. Always question your doctor about potential risks before taking this medication for an off-label purpose.


Many serious side effects may occur when taking Effexor, including increased blood pressure, serotonin syndrome, allergic reactions and changes in mental state. People with high blood pressure or known allergies to the ingredients in Effexor should avoid taking it. Although rare, serotonin syndrome can be fatal. Symptoms of serotonin syndrome include nausea, vomiting, diarrhea, rapid heartbeat, hallucinations, sudden blood pressure changes, fever, shivering, chills, sweating, overactive reflexes, confusion, seizures and a coma. Seek immediate medical attention for anyone displaying these symptoms while taking Effexor. In addition, some people, particularly teenagers or young adults, may experience a negative change in mental state while taking Effexor. This may include worsening depression or anxiety, panic attacks, agitation or irritability, mania, aggressiveness, severe restlessness and thoughts of self-harm or suicide. People experiencing these mental side effects need the immediate attention of a mental health professional or doctor.

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