Drugs can be administered in several formulations via the rectal route. Suppositories are normally either solid suspensions or solid emulsions, whereas rectally administered gelatin capsules can contain liquid formulations. Micro-enemas have a volume of between 1 and 20 ml, and macro enemas 50 ml or more, both of which may be administered as either solutions or suspensions. The suspension suppository is the most widely used formulation, and it has been demonstrated that the release characteristics are dependent upon physiological factors, physicochemical properties of the drug, the suppository base and local environment within the rectum. In general, aqueous solutions of drugs are absorbed more quickly from the rectal route than the oral route, but absorption is usually slower with non-aqueous formulations, due to the limited amount of water available for drug dissolution.
There has been some work exploring the use of controlled release to the rectum, to achieve prolonged and sustained drug delivery. The studies were performed using an osmotically driven device with zero-order release characteristics. It appears to be a promising delivery system for drugs such as nifedipine which effectively reduced blood pressure without the unwanted side effect of increased heart rate115. Hydrogel systems with near zero-order delivery have also been used to deliver morphine intrarectally116.
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