The free base form of mPEG7-0-naloxol has not been observed to form a crystalline phase even when cooled to -60 °C and has been observed to exist as a glass with a transition temperature of
approximately -45 °C. Furthermore, mPEG7-0-naloxol in its free base form can undergo oxidative degradation upon exposure to air. Care can be taken in handling the free base, for example, storing it under inert gas, to avoid its degradation. However, a solid form of mPEG7-0-naloxol, preferably one that is stable when kept exposed to air, is desired