Adults:
The usual initial dose is 15 to 30 mL daily. The dose may be increased to 45 mL daily if necessary. Since lactulose solution relieves constipation by producing a physiological change in the colon, it may take from 24 to 48 hours before normal defaecation occurs. After three days, the dose may be reduced to 10 to 25 mL daily for maintenance

Children:
It is recommended that if lactulose is given to infants and children, this should be done under medical supervision. The usual initial daily doses (for the first three days) are as follows:
- 7 to 14 years: 15 mL
- 1 to 6 years: 10 mL
- Infants under 1 year: 5 mL
Maintenance daily doses are as follows:
- 7 to 14 years: 10 mL
- 1 to 6 years: 5 to 10 mL
- Infants under 1 year: 3 to 5 mL

As a Prebiotic:
Doses of 2g per day have been demonstrated to increase short-chain fatty acid production in vitro. 5g per day has demonstrated a prebiotic effect producing higher counts of Bifidobacterium, Lactobacillus and Anaerostipes with a concomitant rise in branched-chain fatty acids, decrease in branched-chain fatty acids and, pH and ammonia (in vitro model of human large intestine). In vivo studies have demonstrated a bifidogenic effect at a dose of 10g. An initial slow dose and a gradual increase is recommended to prevent side effects such as bloating and flatulence.

500ml

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.