Is there anyway to help reduce the amount of weight gain and water retention while taking prednisone? i will be on it for a week or so for a ms flair

See details: A one week course should not cause an major fluid retention. Any should resolve once the drug is stopped. If it is an issue a diuretic will help.

You bet: Couple approaches: decrease sodium intake and use low salt/ low fat MS diet, increase potassium concurrently. Consider transient use of a diuretic such as hydrochlorothiazide. (Incidently, have been very disappointed with Tecfidera, and find far greater efficacy with Gilenya or Tysabri (natalizumab).)

Not too concerned: If the duration of just one week, the effect will usually not be noticeable. Beware that you may have a greater appetite from it and thus try to control your food intake and perhaps do more exercise. As for the sodium retention, it is not an issue for most people but restricting your salt intake and increasing potassium intake may occasionally be required.