Metoclopramide is a drug which has experienced worldwide use in the clinic for over 30 years as an antiemetic. Recently, it has also been shown to possess radio- and chemosensitizing properties in both animal tumour models and humans at the higher dose of 2 mg/kg. Two new metoclopramide formulations are being clinically developed and they differ mainly in whether the pH of their formulations are acidic (pH 2.5-3.5) or neutral (pH 6.5-7.0). Here we report that intramuscular administration of neutral metoclopramide is about 100% bioavailable, safer and with reduced side effects compared to acidic metoclopramide delivered by intramuscular injection to rats within the dose range of 3.5 to 14 mg/kg. The intramuscular administration of metoclopramide was also about 100% bioavailable compared to the intravenous route of administration. Furthermore, neutral metoclopramide had significantly decreased affinity for dopamine D2 receptors and increased affinity for 5-hydroxytryptamine, receptors, but the radiosensitizing potency was the same, when compared to equimolar concentrations of acidic metoclopramide. Taken together these data support the continued development of neutral metoclopramide for high dose intramuscular administration of metoclopramide for future clinical use as both an antiemetic and radiosensitizer.