A medicine is any substance that is designed to prevent or treat diseases and a drug is designed to produce a specific reaction inside the body. While there is considerable overlap between the two types of substances, these differences are also quite important.
Most of the medicines that are also drugs are considered "controlled substances." This means that there are laws governing their use and that using them in ways contrary to those laws can lead to criminal charges. Antidepressants like Lexapro are drugs, in that they are designed to help alleviate the physical symptoms of depression. However, they are also used in the treatment of the chemical imbalance that leads to depression, so Lexapro is also a medicine. Cocaine, on the other hand, is a drug designed to create a specific mental reaction that leads to a "high" for the user. However, the medical establishment does not recognize any medical benefits for cocaine at this time. Over-the-counter anti-inflammatory medicines such as Advil are designed to treat pain, but they do not have a strong enough effect to fit into a controlled substance classification, unlike stronger pain relievers. This means that these are medicines rather than drugs. Understanding the similarities and differences between drugs and medicines is an important part of medical and pharmaceutical training.