Drug effectiveness is the key measure of whether any pharmaceutical product is permitted to be marketed to the public. A drug is defined in two separate, but related, ways. The first is that it is a substance employed in the diagnosis, treatment, or prevention of disease, while the second definition is that a drug is any chemical that acts on the central nervous system to produce a change in mood or behaviors. Drugs are often the subject of legal definition in legislation regulating their manufacture or sale.
The effectiveness of a drug is closely linked to its safety. It is for this reason that the legal approval for the administration of a particular drug is assessed on a risk/benefit analysis. If a drug is known to generate significant side effects in a consumer, it will have a reduced effectiveness, notwithstanding the drug's ability to counter a particular physical condition.
Aspirin is an example of a drug whose effectiveness was reconsidered over time, as it was found to have applications to conditions other than its original purpose—the relief of pain. Acetylsalicylic acid (ASA), the active ingredient in aspirin, had been employed as an herbal remedy (willow bark) for centuries prior to the synthesis of ASA in 1898. A nonprescription, nonsteroidal anti-inflammatory drug (NSAID), aspirin in the 1980s began to be recommended by physicians to reduce the risk of heart attack and stroke, as ASA was proven to have blood-thinning properties. The range of aspirin's effectiveness as a drug was greatly expanded.
With the expanded range of aspirin applications came newly identified risks, the chief of which related to stomach irritation and the potential formation of ulcers. The effectiveness of aspirin, the most consumed pharmaceutical product in North America, is therefore weighed against the risk presented to each individual user.
Beta-blockers are substances administered to persons at risk of heart attack, acting to slow the heart rate and to reduce blood pressure. While highly effective in this role, the use of such drugs is prohibited through the World Anti-Doping Code, rendering the substance ineffective for a competitive athlete seeking such a physical advantage.
Another aspect of drug effectiveness is the increased tolerance that the body sometimes develops over time. A number of corticosteroid anti-inflammatory drugs, which rely on the body's systems being triggered into action, become less effective over time.