@Kinky_John
Sure. Lactulose is pretty much a sugar syrup and dissolves in any watery drink… just like treacle, molasses, maple syrup, etc… .
It's just another sugar, but one that humans cannot digest, because it doesn't occur naturally so we don't have the needed enzyme. So it goes though you and into your large bowel. Most any undigested sugar in there irritates the bowel wall and it wants to be rid of the irritating stuff… pronto… so you poo. The longer it stays in there the more powerful the need to poo.
The dosage you cite is about right. A somewhat smaller or larger dose won't much alter the results. Smaller doses (15ml) are sometimes given daily to ‘keep someone going'.
Basically, if you take your dose you should poo, sooner or later. The longer you resist the softer, more fluid, the poo will become. Sooner or later, even if the recipient is packed with hard poo, the combination of fluid in the bowel and the massaging of the camps, will turn the poo into pudding and a flood is inevitable.
Some of the sugar is digested by colonic bacteria with gas as a reaction product. When you start to fart, it is a waning that a flood of poo is going to follow quite soon.
The combination of soupy poo and a lot of gas can result in a petty spectacular, noisy, messy poo.