Yes after all most pet drugs are originally made for humans so you can just give the dog a smaller dose (as much as you would give to a 30kg child) and don't forget to read the ingredients at the back of the box and make sure that nothing is poisoners to the canine.
Be sure to check that your dog doesn't have diarrhoea. A common mistake among most owners is thinking that their dogs is constipated when they just have some diarrhoea. If it continues for much longer, I would take him to the Vet. There could be an object that he as eating in there blocking everything up which may require surgery. Another tactic to defeat constipation is try feeding him a little (about 2 oz) of mineral oil along with wet dog food. Should sicken things up in there right away! :)