Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
4. Health and Safety
Why are some medicines liquid and some medicines pills?
Besides preventing people from misusing certain drugs, liquid drug delivery systems are sometimes preferred because they are faster acting than pills. An extreme example is nasal spray, which acts very quickly because it can be delivered right into the nose and then to the brain, without first having to be digested in the stomach and intestines to get into the bloodstream.
Pills make bad tasting medicines easy to swallow, since only a little of the medicine actually touches the tongue before the pill is swallowed. But pills and capsules can also be designed to pass through the acidic stomach without being harmed, so they can dissolve in the alkaline intestines and be absorbed into the bloodstream. Stomach acids can break down some medicines and prevent them from doing their job.
Pills and capsules can also be designed to dissolve slowly, so that they release small amounts of the medicine over a long period of time. This allows the patient to take one pill and get relief from symptoms for a whole day.
Some things we take in pill form are liquids. An example is vitamin A. It can be absorbed into powder to make a pill, but it can also be enclosed in a soft gelatin capsule that dissolves in the stomach to release the oily liquid vitamin.