The most common side effects with Xenical (seen in more than 1 patient in 10) are influenza (flu), hypoglycaemia (low blood sugar levels), headache, upper respiratory infection (colds), oily spotting from the rectum, abdominal (tummy) pain or discomfort, flatus (gas) with discharge, faecal urgency (urgent need to open the bowels), fatty or oily stools, flatulence (gas), liquid stools, oily evacuation (faeces) and increased defecation.