Adverse event: an undesired action or effect of an experimental treatment (side effect).

Cure: see sustained virological response.

Efficacy: refers to the drug’s effectiveness i.e., if it works.

Exclusion criteria: conditions that disqualify someone from participating in a clinical trial. Food and Drug Administration (FDA): The federal government agency that is responsible for granting or denying approval for drugs to be sold to the public. This U.S. government agency has many other functions.

Genotype: genetic variations (strains) of the hepatitis C virus. There are seven genotypes of hepatitis C. There are also many subtypes such as genotype 1a, 1b, 2a, etc. The majority of people in the United States are infected with genotype 1 (70%) followed by genotype 2 and 3 (30%).

HCV RNA: the genetic material of the hepatitis C virus. HCV is a single-stranded ribonucleic acid (RNA) virus.

Interventional study: an experimental study to test a new medicine to treat a certain medical condition.

Inclusion criteria: conditions that must be met in order to be eligible for a clinical trial.

Longitudinal study: a term used for studying people in a clinical trial over a period of time.

Observational study: a study about the possible effect of a treatment on the study participants.

Open label study: a study when both the study researcher (medical providers) and the study participant know the drug(s) the study participant is receiving.

Phase I study: the initial introduction of an investigational new drug using humans. The subjects in this phase are usually healthy volunteers. Sometimes the subjects are those with the disease that is being studied. The goals of a Phase I trial are to evaluate safety and tolerability (i.e., lack of major side effects) as well as the dosage range. This is determined by testing a range of doses (called a dose-ranging trial). Study participants initially receive a low dose of the drug; this is gradually increased as long as the drug appears to be safe. Phase I studies may provide early indications of the drug’s effectiveness, but whether or not a drug works is the primary focus of Phase II and Phase III studies. A small number of patients are enrolled in the study.

Phase II studies: studies that are conducted using patients with the disease for which the drug is being tested. The goal of this phase is to obtain preliminary data on the effectiveness (also known as efficacy) of the test drug. This phase also allows further collection of data on the common short-term side effects and risks associated with the drug. A relatively small number of participants enroll in Phase II studies.

Phase III studies: the goal of Phase III studies is to gain additional information on effectiveness and safety. In this phase, several hundred to several thousand subjects receive the test drug. In Phase III studies, the new drug is often compared to current standard therapy. If the phase III trials are successful the FDA may approve the drug to market it for the treatment of that condition.

Phase IV studies: After the drug is approved and marketed, the FDA may require a company to obtain more information about the drug. These studies occur as Phase IV trials. Examples are the safety and efficacy of varying doses, how the drug interacts with other drugs, or how it works in people with other diseases. Phase IV trials may include small or large numbers of subjects and may reveal uncommon side effects that are too rare to show up in Phase II or III studies.

Placebo: a pill (capsule, liquid, or injection) that contains an inactive substance. It is compared to the experimental drug in placebo-controlled clinical trials. Also called a sugar pill.

Randomized study: a study that randomly assigns (using a computer) study participants into a experimental group or a control group.

Safety: refers to the drug’s toxicity or side effects.

Sponsor: a sponsor of a study is typically a drug or biologic manufacturer. Other potential sponsors include a university or independent foundation that supports the research.

Standard treatment: the best or most widely used currently available treatment. Sustained virological response (SVR12): undetectable HCV RNA (viral load) 12 weeks after treatment has ended. Also called viralogic cure or simply cure.

Treatment-experienced: a person with hepatitis C who has been previously treated but who has not been cured of hepatitis C.

Treatment-naïve: a person with hepatitis C who has never been treated.