Drugs in Daily life
A drug is a chemical that interacts with proteins in the body to affect a physiological function. This is the general idea behind all medicine. Once these chemicals are absorbed into the systemic circulation they bind with certain proteins and this changes the functioning of the cell slightly. For example, anticancer drugs bind to proteins on the surface of cancer cells this stimulates the cells to die. In this case cell death is the physiological action of the drug.
No drugs are specific to interacting with just one type of cell or one type of protein and this is what causes side effects. Again using an anticancer drug as an example, the medication works by binding to very rapidly dividing cells, such as cancer cells, however hair cells are also rapidly dividing and that is why one of the side effects of anticancer drugs is hair loss.
The chemical in the drug that affects physiological functioning is the active ingredient of the drug. For most drugs, the amount of chemical needed to cause an effect is very small, often as small as 5 micrograms; this is 0.005% of a gram! As you can imagine this is too small to package and handle, these ingredients are very expensive and giving out little amounts like that will cause most of the drug to be lost and wasted. Therefore most of the drugs that we take are also comprised of inactive ingredients that work to fill out the drug. Inactive ingredients are, as the name suggests, ingredients that have no effect on the functioning of cells, namely lactose, dyes and gluten. If the drug needs to be taken orally, the inactive ingredients also work to bind the drug together and lubricate the drug so it is easy to swallow.
So the inactive ingredients are the fillers, binders and lubricants of the drug whereas the active ingredient is the very small amount of chemical that reacts with the body to produce an effect.