The 50 mg. arrived. Now the 25 mg. is on backorder. I was wondering if worse comes to worst can I substitute the 25 mg. with a generic trimipramine? Also what medications are close in chemistry to Surmontil? Also, if I were to receive another type of antidepressant, is it difficult changing from one medication to some other type of antidepressant similar in makeup chemistry as the one I can no longer obtain?