The dose of a drug is tailored to the amount that will elicit a desired response and is tolerated without excessive side-effects. This dose varies from person to person and may depend on the weight and sex of the patient. Kidney and liver damage may delay excretion, and a reduction in dose may be needed to avoid toxic effects. Tolerance to a drug may be acquired after repeated doses, and then a larger drug dose may be needed to elicit the same response. Care should be taken when giving drugs to children. Their liver enzyme system may not be fully developed and an adult dose may result in toxic effects. In some children and adults there is a totally unexpected response to a drug, a reaction known as idiosyncrasy, which may be associated with a genetic abnormality. Hypersensitivity to drugs is also a serious problem. It is usually allergic in nature and similar to the antibody-antigen reaction seen after infection.
Aspirin, for example, may start an asthma attack in hypersensitive patients.
Side-effects can also cause problems. No drug has an effect solely on the desired organ or disorder. For example, the powerful pain-killer morphine also induces nausea and vomiting.
Side-effects are defined as physical responses that can be tolerated. When they are more unpleasant they are known as adverse reactions, and a subsequent reduction in dose or withdrawal of a drug may be necessary. Adverse reactions can include liver damage and even death, and drugs that have cost a great deal of money to develop occasionally cause too much harm to too many people to allow them to continue being marketed.
Before a drug is allowed onto the market it has to go through rigorous animal and human-volunteer testing to assess its safety and effectiveness. Some of the subjects in such tests may be given a placebo, an inactive substance that looks like the active drug. Placebos that look like medicines are also sometimes given by doctors to individuals who mistakenly believe that they need medicines.