Because of advancements in the analytical methods used for detecting chemicals in the environment, trace levels of prescription and over-the-counter drugs have been found in waterways. The frequent detection of many pharmaceutical compounds in the environment has been an increasing concern because of their potential to cause undesirable ecological effects, which may range from endocrine disruption in fish and wildlife to antibiotic resistance in pathogenic bacteria. Residues of human and veterinary pharmaceuticals (such as antibiotics, estrogens, and active ingredients of drugs) are introduced into the environment via a number of pathways, but primarily from discharges of wastewater treatment plants or land application of sewage sludge and animal manure. Most active ingredients of pharmaceuticals are transformed only partially in the body and thus are excreted as a mixture of metabolites and bioactive forms into sewage systems.