depression
"Though never clinically depressed, I discovered the powerful side effect of tramadol while taking 150 mg/day for chronic sciatica.  The unrelenting pain was a real downer, but the tramadol reliably helped me maintain an optimistic, upbeat attitude.  During five years of taking it, I never had to increase the dosage for the desired effect.  The downside is that it often interfered with sleep so that I found a mid-day nap essential."