I was on 7.5 mg mirtazapine 3 years ago, hopeful it would help my digestive system and anxiety and insomnia but I had the exact same problem. Absolutely lethargic during the day from exhaustion. I went off of it because of the side effects but I did learn afterwards that mirtazapine is an unusual drug in that the tired feeling, the sleepiness effect is much stronger in a lower dose than in a higher dose. Cut to this past November and my new psychiatrist suggested trying it for my anxiety and insomnia and since I already knew that 7.5 mg was a miserable experience, I asked her if we could try 15 mg , which we did, and it's been a completely different experience. It's helping my digestion and my anxiety. It did take a few weeks for me to get used to that dose for insomnia, leading to nights of only being able to sleep in increments of 4 hours, but now I'm completely adjusted and it's been a very beneficial medication for me.