15mg of prednisone is a low dose. If you adjust what and how much you eat you will not gain weight. Patients do not act and take high doses of prednisone and have reduced activity which is a formula for gaining weight. I was on 60mg and have been on lower doses for over 3 years and have zero weight gain. Also have a severe case of polymyositis and lost 40 lbs of muscle mass 3 years ago so had far fewer muscles to burn calories. Spent over 3 months completely crippled and in hospitals but have been able to maintain my predisease weight now for almost 3 years.
Make some adjustments and you will be OK on such a low dose.
Others have already given you good advice. Don't play doctor and change your meds. Controlling your immune system is not like taking pills to cure an infection. Some of us will have to take some dosage of controlling meds for life. Others will be able to taper off but it can take many months or even several years until your immune system stabilizes. EVery case is different and you must have patience. Coming off meds too soon is one of the biggest mistakes you can make because you risk having a flare and never find a maintenance dose to control your disease.
If you are proactive you can avoid weight gain but be aware of any symptoms you might have and tell your doctor. If you get the infamous moon face or prednisone belly that will go away when you taper to a lower dose or off the meds. At 15mg you might not have any problems at all.
Mixed connective tissue disease (systemic lupus, scleroderma, polymyositis), Raynauds phenomena, Hypertension, Barrett's esophagus.
Meds: prednisone 5mg, 50mg imuran, Lotrel 40/5mg, maxide 37.5/25mg, folic acid, potassium, aciphex 20mg, multi vitamins.
Maintain your optimism and you can beat the odds.
"Although the world is full of suffering, it is also full of the overcoming of it." Helen Keller