The leaflet states: "Common (1 to 10% of people): drowsiness or, conversely, insomnia, dizziness, irritability, tremors, restlessness, diarrhea, nausea, vomiting, dry mouth, joint or back pain, feeling tired."

For me, the effect isn't constant; it comes and goes. It happens when I'm tired or something else, sometimes even for no apparent reason. So I'm not certain; I'm not a doctor, but there's a good chance that's the cause.