From the point of view of a drug, it's a long trip from the pill bottle to an ache or the site of an infection.

After someone swallows a medicine, the chemical must traverse a veritable maze. It has to survive a journey through the stomach and reach the intestines intact before crossing the intestinal wall into circulation. Once in the blood, it gets filtered through the liver before it can travel to the rest of the body. At each "way station," the compound must resist the acids of digestive juices, jump membrane barriers or fend off enzymes designed to chop it into useless bits.

This article was originally published with the title Where a Pill Won't Reach.

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