I am on pristiq (desvenlafaxine) 50 for anxiety and depression. it is day 13, my mood seems to fluctuate from energized to flat, tired and unmotivated.

Too early to tell: In general, antidepressants take about 4-6 weeks before an appreciable difference is seen. However, during the early stages of treatment, it is also possible that the symptoms you describe may be side effects of medication. Recommend discussing your concerns with your doctor, who knows your particular history. From there an appropriate clinical decision can be made on how to proceed. Best.

This is something: that you should inform your prescribing physician about if you're still feeling this way by day 21. Meanwhile, continue taking it as prescribed. Best wishes. Dr. Anne.