The most common type of medicine has a chemical compound as its active ingredient. These compounds are referred to as small molecules because they are significantly smaller than biological molecules like proteins. They are usually taken by mouth in the form of a tablet, capsule, or liquid, but they can also be injected or infused.
Small-molecule medicines may provide the best or only way to treat certain diseases. Because of their size, these compounds can pass through cell walls to engage targets inside the cell. They can also be designed to cross the blood-brain barrier and engage targets that may contribute to neurological illnesses.