Milnacipran is used to treat fibromyalgia (a long-lasting condition that may cause pain, muscle stiffness and tenderness, tiredness, and difficulty falling asleep or staying asleep). Milnacipran is in a class of medications called selective serotonin and norepinephrine reuptake inhibitors (SNRIs). It works by increasing the amount of serotonin and norepinephrine, natural substances that help stop the movement of pain signals in the brain.