Side affects Mirtazipine/Diazepam

My partner is currently on 30mg Mirtazipine and has been on this for almost 10 years. Recently he was prescribed diazepam for really bad anxiety, 2.5mg for a week. Since then, he has had the most horrendous side effects, slurring his words, drowsiness, sickness, nightmares, sweating. He spoke with his GP who has now reduced the mirtazipine to 15mg for 2 weeks and then changing him to sertraline going forward. Are these side affects normal, its horrific to watch him like this

I’m sorry that your partner is feeling so unwell. Anxiety and depression are terrible, not just for the person who is poorly, but for family too. I live in that dark space, alone, and it can be very frightening. I wish I could offer some advice and support; but send you all my love instead.