If preservatives are a concern to the patient, it may be in the patient's best interest to consult with the physician to determine the duration of therapy. You, as the dispensing pharmacist may elect to dispense the medicated suspension in quantities of only a few days. In this event a preservative may not be needed provided the medication is stored in the refrigerator. The use of the common parabens decrease the susceptibility of microbial attack. Both of these parabens dissolve slowly and may require use of boiling water to speed up the process.
Stability is still another consideration.
Both physical and chemical stability will probably be aided by refrigeration. Chemical degradation, for a variety of reasons, is favored by elevated temperatures. In the event your suspension is difficult to resuspend, consider an oversized bottle, especially if you have a viscous product. The first step is choosing a delivery dose. Next is the choice of vehicle.