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When a drug is swallowed, ingested or absorbed through the skin, in most cases it will enter the bloodstream and circulate throughout the body interacting with a number of target sites. In some cases a drug may act in only one specific part of the body, depending on the properties and administration. In either case, the interaction with the target site will usually produce the desired effect, even though interaction with other cells, organs and tissues may result in what we know as side effects. Pharmacodynamics is a description of the many different ways that drugs affect the body.

While some drugs act on many different tissues and organs, others affect mainly a single organ or system. Atopine is given to relax muscles in the gastrointestinal tract but can also relax the muscles of the eye, respiratory tract and decrease the secretions of the mucous glands and sweat glands. On the other hand, digitalis, which is given for heart failure, works on the heart to increase its pumping efficiency. The answer to how drugs know where to exert their effects lies in how they interact with cells or enzymes. In many cases a drug will bind to cells by means of receptors on the surface of the cell. This allows the activity of the cell to be influenced by chemicals like drugs or hormones that are located outside the cell. The specific configuration of the receptor allows only a drug which fits precisely to attach to it.

Although some drugs attach only to one type of receptor there are others that can attach to several types of receptors throughout the body. It is believed that nature did not create receptors since they have a natural purpose but that drugs take advantage of receptors. For example, pain relieving drugs attach to the same receptors in the brain as do endorphines. There are a class of drugs called agonist which activate or stimulate their receptors to trigger a response that will either increase or decrease the cell's function. A second class of drugs called antogonist will block the access or binding of agonist to their receptors. In some cases such as asthma, agonists and antagonists are used in different but complimentary approaches to treatment. Another important target for drug action are enzymes. Enzymes help transport vital chemicals, regulate the rate of chemical reactions or serve other transport, regulatory or structural functions. Drugs targeted at enzymes are classified as inhibitors or inducers.

Affinity and intrinsic activity are also two drug properties that are important to drug activity in the body. The mutual strength of the bond between a drug and its target is called affinity. The measure of the drug's ability to produce a pharmacologic effect when bound to a receptor is called intrinsic activity. The potency of a drug refers to the amount of drug needed to produce a given effect. The efficacy of a drug refers to the potential maximum therapeutic response which a drug can produce. Drug tolerance refers to what occurs when the body adapts to the continued presence of a drug. Although many of the drugs that are currently being used were discovered by experimental trial and observation, it is good to note that no drugs are perfectly effective and completely safe. Due to this it is important that a doctor assess the drugs benefits and risk with every situation that requires prescription drug treatment.