Initial dosing of lactulose may cause flatulence and abdominal cramps which are generally transient. Excessive dosage can lead to diarrhea with excessive loss of water and electrolytes (particularly potassium and sodium). Hypernatremia, nausea and vomiting have also been reported.
Some consider the taste unpleasant. This can be minimized by dilution of water, fruit juice, or milk, or by mixing the dose with food.