Your Wife maybe suffering from Depression as these are common antidepressant which have been prescribed to you wife.
Suicidal ideas is a symptom of Depression and it may worsen as medicines take upto 2 weeks to be effective.
Mirtazapine does cause Drowsiness but gets better with time.
Next Steps
Continue the meds as advised. Consult online if in Dilemma.
Health Tips
Dont stop medicine suddenly.