Quick Answer
Because antidepressant monotherapy in bipolar disorder can trigger manic or hypomanic episodes, worsen mood cycling, and delay effective stabilization. Bipolar disorder is a mood-regulation problem, not just a low-mood problem — the medications that work for it (lithium, valproate, lamotrigine, atypical antipsychotics) target mood stabilization. Antidepressants can be used in bipolar care, but almost always alongside a mood stabilizer, and not as standalone treatment. This is why we screen every depression patient for bipolar features before starting an antidepressant.
This question comes up frequently for patients researching bipolar disorder treatment at Good Health NC in Knightdale, NC.