Brief Answer: Serious interactions present. Detailed Answer: Hi, Thanks for writing to us, Effexor is venlafaxine which is a SNRI or serotonin norepinephrine reuptake inhibitor. It increases the level of serotonin and norepinephrine in the body by preventing their reuptake at the nerve endings. It is used to treat depression commonly. Usual side effects include, nausea, headache, insomnia, somnolence, dry mouth, dizziness, constipation, asthenia, sweating, and nervousness. It can also cause a sustained increase in blood pressure on a long term. An overdose can cause, electrocardiogram (ECG) changes, tachycardia, bradycardia, hypotension, hypertension, coma and seizures. Phentermine is an amphetamine derivative and is similar to its parent drug in many aspects. It suppresses appetite centrally and increases norepinephrine levels. It additionally increases levels of serotonin and dopamine. It is used as an appetite suppressant in obese individuals. It has a potential of causing dependence in the body if taken regularly because of which it requires strict supervision. It has certain side effects of anxiety, palpitations, tremor and insomnia. If taken in large amounts it can be detrimental to the heart. The interaction between these two drugs can be potentially life-threatening. Due to a common mechanism of action of increasing serotonin, it has the potential of causing what is called "serotonin syndrome" where there is excess serotonin in the body and can manifest as headache, agitation, mental confusion, hallucinations, coma, shivering, sweating, increased body temperature, hypertension, tachycardia, nausea, diarrhea and seizures. It is rare but chances may increase with such a combination of drugs. One drug may also cause toxicity of the other. This combination may be prescribed by doctors after a detailed evaluation and in selected cases if there are no other options. In such cases it is to be taken under the supervision of the doctor. Hope this helps, Dr A Rao

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