insomnia
"I suffered a neurotoxic reaction to another drug, metronidazole; it caused lesions for form on my brain.  This led to neuropsychiatric symptoms, including severe anxiety, major depression, suicidal thoughts (just a month before, I was happy), and horrible insomnia.  I was given mirtazapine and it greatly helped, especially the insomnia.  I had barely slept for eight days--even  Ambien couldn't knock me out after the first night (I'd never taken a sleeping pill in my life and even Ambien couldn't help).  Mirtazapine was a life-saver, literally.  Metronidazole had overstimulated my central nervous system and mirtazapine helped to calm it down.  It took longer to treat the other symptoms, but they, too, resolved."