Lactulose can cause side effects, but most people will usually tolerate the medicine well. A very common side effect of lactulose is diarrhoea, particularly if you take a high dose of the medicine. Other side effects include bloating, wind, feeling sick, being sick and stomach pain. These side effects are typically mild and will usually go away after a couple of days. If you suffer from severe diarrhoea or vomiting for more than a couple of days, and you start to experience muscle cramps or an irregular heartbeat, these could be signs of an electrolyte imbalance and you should seek medical attention immediately.