Prednisone changes the way we metabolise food. I had constipation in the early days of prednisone and increased the water I was drinking and made sure to include a couple of tablespoons of extra virgin olive oil with a big salad every day. Fresh vegetables rather than high sugar fruit have been essential for me to stay regular and avoid weight gain. Physical inactivity can also contribute to constipation. Pay attention to your body and what you eat, keep active, and drink plenty of water.