Depending on product type and development stage, investigators initially enroll volunteers and/or patients into small pilot studies, and subsequently conduct progressively larger scale comparative studies. As positive safety and efficacy data are gathered, the number of patients typically increases. Clinical trials can vary in size, and can involve a single research entity in one country or multiple entities in multiple countries.

In 2012, Z. Janet Yang, Katherine A. McComas, Geri K. Gay, John P. Leonard, Andrew J. Dannenberg, and Hildy Dillon conducted research on the attitudes towards clinical trial treatment and the decision making of signing up for such trials by cancer patients and the general population. They used the risk information seeking and processing (RISP) model to analyze the social implications that affect attitudes and decision making pertaining to clinical trials. People who hold a higher stake or interest in clinical trial treatment showed a greater likelihood of seeking information about clinical trials. Those with networks that stress the importance of learning about clinical trials are also more likely to seek and process information more deeply. People with more knowledge about clinical trials tend to have to a greater likelihood of signing up. In the study, cancer patients reported more optimistic attitudes towards clinical trials than the general population. Having a more optimistic outlook on clinical trials also leads to greater likelihood of enrolling.

The Human Genome Project originally aimed to map the nucleotides contained in a human haploid reference genome (more than three billion). The «genome» of any given individual is unique; mapping «the human genome» involves sequencing multiple variations of each gene. The project did not study the entire DNA found in human cells; some heterochromatic areas (about 8% of the total genome) remain unsequenced.

Most gross genomic mutations in gamete germ cells probably result in inviable embryos; however, a number of human diseases are related to large-scale genomic abnormalities. Down syndrome, Turner Syndrome, and a number of other diseases result from nondisjunction of entire chromosomes. Cancer cells frequently have aneuploidy of chromosomes and chromosome arms, although a cause and effect relationship between aneuploidy and cancer has not been established.

The removal of ethanol (drinking alcohol) through oxidation by alcohol dehydrogenase in the liver from the human body is limited. Hence the removal of a large concentration of alcohol from blood may follow zero-order kinetics. Also the rate-limiting steps for one substance may be in common with other substances. For instance, the blood alcohol concentration can be used to modify the biochemistry of methanol and ethylene glycol. In this way the oxidation of methanol to the toxic formaldehyde and formic acid in the human body can be prevented by giving an appropriate amount of ethanol to a person who has ingested methanol. Note that methanol is very toxic and causes blindness and death. A person who has ingested ethylene glycol can be treated in the same way. Half life is also relative to the subjective metabolic rate of the individual in question.

In some US locations, the local IRB must certify researchers and their staff before they can conduct clinical trials. They must understand the federal patient privacy (HIPAA) law and good clinical practice. The International Conference of Harmonisation Guidelines for Good Clinical Practice is a set of standards used internationally for the conduct of clinical trials. The guidelines aim to ensure the «rights, safety and well being of trial subjects are protected».