Lactulose is taken orally and reaches the large bowel without being absorbed into the bloodstream. Here, it is degraded by the normal bacteria found in the bowel, and this changes the pH, making it more acidic. This has two effects, one is to increase the amount of water in the bowel, which makes the stools softer and allows defecation to occur. The other is to remove harmful ammonia from the system and expel it in faeces, preventing the brain from being exposed to it as happens in PSE.
Common side effects are distension, flatulence and intestinal cramps occur in 20% of cases, which are usually mild and transient. Excessive dosage can lead to diarrhoea, which can lead to fluid loss and electrolyte imbalance if left untreated.
Uncommon side effects include nausea, vomiting, anorexia and increased thirst.