If you are planning to stop Sertraline, the dose of Venlafaxine can be increased further by your doctor depending on the clinical response.
Both Venlafaxine ER and Effexor ER are the same medicines [ Effexor ER is the brand name of Venlafaxine ER.]
The usual dose range of Venlafaxine ER is from 75-225 mg per day and the maximum dose is 375 mg per day.
You are already taking 225 mg of Venlafaxine ER (150 + 75) so if you plan to taper down and stop Sertraline, either your doctor can further increase the dose of Venlafaxine ER by 37.5 mg / 75 mg or can consider the option of adding another antidepressant like Mirtazapine or Bupropion to augment the effect of Venlafaxine ER.
Since both Mirtazapine and Bupropion are not SSRI antidepressant like Sertraline so chances of Serotonin syndrome with the combination of Venlafaxine + Bupropion / Mirtazapine are none or minimal.
In my view, you should discuss with your doctor about replacing Sertraline with either Bupropion (if you have additional symptoms such as lack of concentration, fatigue, poor memory) or with Mirtazapine (if you have additional insomnia
) instead of increasing the dose of Effexor further.
Please feel free to discuss further if you have any doubt / query regarding my suggestion.