If you were to take double the amount of prescribed Effexor by mistake what are the possible side effects?

Symptoms may include tiredness, mild convulsions, and cardiac arrest, but this would be with a dosage considerably higher than double your prescribed amount.

Also, since the dosage varies from person to person, depending on the severity of their condition, your normal dosage may be well below the highest recommended dosage.

If you are still concerned that you may suffer ill effects from taking a double dose, you can call your pharmacists, who can tell you if the amount you took warrants a call to your doctor. I've found that it's much quicker getting to talk to a pharmacist than it is to a doctor, which is why I would call my pharmacist first. Pharmacists are extremely well educated as to the side effects of drugs, and can give Excellent, accurate advice.