Thank you for your query,
is a syrup given to treat constipation
and it is not digested by the gut.
Lactose is a sugar that is present in milk of humans, cows and in the formula milks and it will be digested by the gut.
Farex milk contains lactose and not the lactulose.
Some times infants may develop secondary lactose intolerance
so whenever they fed with lactose formula they cant digest the lactose and they develop loose stools.
At least for 8 weeks duration you have to give lactose free milk in secondary lactose intolerance.
So consult your Pediatrician
regarding the feeding advise.
Hope I have answered your query.