https://www.quora.com/Why-dont...wallow-suppositoriesThere are a couple reasons a drug might be delivered as a suppository rather than in an oral formulation. The most interesting reason has to do with human anatomy:
As you probably learned early in life, arteries bring blood away from the heart to the tissues that need it, and veins bring blood back to the heart after it has been depleted of oxygen and nutrients by the tissue. However, in the GI tract, there is a special circulatory system, called the “hepatic portal system,” that works a little bit differently. After going to your gut, the blood doesn’t return immediately to your heart - it first makes a stop at your liver.
This is a very good thing for every day life. One of the liver’s jobs is to detoxify various compounds that we regularly eat that would otherwise be poisonous for us. When these compounds are absorbed into our blood, instead of getting circulated to our body the compounds are first brought to the liver where they are detoxified, making them safe to circulate and easier to excrete in the urine. This process is called “first pass metabolism.”
While first pass metabolism is great for getting rid of toxic things we eat, it has the unfortunate effect of also “detoxifying” many of the drugs that we take orally. This means that very little of the drug is actually able to get into your circulation, making the effective dose much smaller than the actual dose. This can be a real problem for achieving the amount required, or maintaining a certain amount in the blood over a period of time.
There are a couple of ways around this. First, we can just inject the drug straight into the bloodstream, or under the skin. This is commonly done in the hospital, but most people aren’t comfortable injecting themselves on a regular basis. Another approach is to try to deliver drugs to tissues that are not a part of the portal system.
As it happens, the veins in the rectum are different than the veins elsewhere in the GI tract. They go straight to the heart, without making a stop in the liver first. This is why some drugs are formulated as suppositories. In order to get the same effect orally, you would have to swallow much, much more of the drug, which is not only unpleasant but could be taxing on your liver as well. -Daniel Starer Stor, MD/PhD Cellular Metabolism
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