Drugs act by specifically interfering with biochemical processes within the body. At first, drugs have to reach their target, i.e. the site where the therapeutic action is required (also called target site). Whatever the application of a drug (oral, intravenous, topical etc.), its journey to the target site is not at all an easy task. The circulatory blood stream and proteins like the human serum albumin and the high density (HDL) and low density (LDL) lipoproteins, usually transport drugs throughout the body. Sometimes, artificial carriers, which are specially designed by chemist and pharmacologists and make part of the drug formulation, facilitate drug delivery.
When a drug reaches its target site, it enters in action, which means that it starts interacting with specific biological substrates such as the proteins, the DNA etc. In the case of DNA, this interaction could result for example, in interrupting the replication process, which eventually leads to cell death (objective of cancer therapy). Nevertheless, drugs delivery to their target sites is not always successful. Further interaction with biological substrates other than their original targets may also happen, which will cause negative side effects. Drug side effects are sometimes unavoidable.