Lot of luxatives are available in market. But formost queation is that why your dog is having constipation or difficulty is passing stool which includes internal parasites, any obstruction in intestine to rectum, low fibre diet, prostate enlargement etc. I recommend to get your dog examined by vet so that reason is identified and management is done accordingky.
Giving unnecessary luxative can lead to loosemotion, electrolyte imbalances, dehydration, intestinal damagr et c.