Runt puppies may need a little extra supervision to thrive. Make sure the puppy is feeding regularly and not being bullied out of food or milk by its larger siblings. Talk to your vet about caring for the runt. They can provide further assessment of the puppy's health, and recommend supplements and strategies for the puppy's care. A few days of good care can make a big difference in an early puppy's health. You'll be surprised by the difference in runt puppies before and after attentive care.