The short answer
- Strength: about 3.3 g per 5 mL (labelled 10 g/15 mL in the US, 3.1–3.7 g/5 mL in the UK, 3.34 g/5 mL in Australia).
- Bottle sizes: commonly 300 mL and 500 mL (UK), 500 mL and 1 L (Australia), 473 mL (16 fl oz) and 946 mL (32 fl oz) in the US; plus 15 mL sachets.
- Constipation dose: usually 15 mL once or twice a day for adults; it takes 1–2 days to work.
Lactulose products and sizes
| Country |
Examples |
Typical pack sizes |
| UK |
Lactulose oral solution (generic), Duphalac |
300 mL, 500 mL; 15 mL sachets; some 1 L packs |
| Australia |
Duphalac, Actilax, generic lactulose |
500 mL, 1 L |
| US |
Generic lactulose solution (Constulose, Enulose), Kristalose powder |
473 mL, 946 mL, larger pharmacy sizes; Kristalose 10 g and 20 g packets |
Exact sizes change with suppliers, so ask your pharmacist what’s available. Prescription quantities are often written in millilitres; a 500 mL bottle at 15 mL twice daily lasts about 16 days, at 15 mL once daily about a month.
How much lactulose to take
| Use |
Typical adult dose |
Notes |
| Constipation |
15 mL once or twice daily, adjusted to response |
Works in 24–48 hours; can reduce once stools soften |
| Hepatic encephalopathy (liver disease) |
30–50 mL two to four times daily |
Aim for 2–3 soft stools a day; prescribed and monitored |
| Children |
Weight- and age-based |
Follow the label or prescriber; use an oral syringe |
Lactulose is an osmotic laxative: it isn’t absorbed, so it draws water into the bowel and is broken down by gut bacteria into acids that soften stool. That fermentation is also why it causes wind and bloating, especially in the first days.
Tips for taking lactulose
- Measure with the cup or an oral syringe, not a kitchen spoon.
- It’s very sweet: mix with water, juice or milk, or follow with a drink.
- Drink plenty of fluids during the day.
- Store at room temperature; don’t refrigerate (it can thicken). Check the “use within” period after opening.
- Diabetes: lactulose contains small amounts of other sugars; usual doses for constipation are generally fine, but higher doses may affect glucose.
Lactulose or another laxative?
For long-term constipation in adults, macrogol (Movicol, OsmoLax, MiraLAX) is often better tolerated, with less wind. Stimulant laxatives (senna, bisacodyl) work overnight and suit opioid-related constipation. Lactulose remains first choice in liver disease. The options are compared in the stomach medicines guide.
Frequently asked questions
How long does a bottle of lactulose last once opened?
Check the label; many products should be used within a few months of opening. Discard it if it changes colour or becomes cloudy.
Can I take lactulose every day?
Yes, it can be used long term if needed, but see a doctor if constipation is new, persistent or comes with weight loss, bleeding or abdominal pain.
Is lactulose safe in pregnancy?
Yes, it is commonly used in pregnancy and breastfeeding because very little is absorbed.
Sources and further reading
CanforaMedicine Guide
