Clinical trials

Clinical trials are the core of the process of testing through which new drugs (and other treatments such as vaccines) go in order to demonstrate their safety and effectiveness to regulators. They are the parts of the testing process that involve administering a drug (or other treatment) to humans. Clinical trials are conducted by the developers of a drug and the data is passed on to regulators.

Clinical trials are a long and complex process that needs to determine not only how safe a drug is, but also how effective it is. It not only important that a drug be approved, but what it is approved for. Regulators restrict the marketing of a drug, and in particular the labelling - the information a pharmaceutical company may distribute about the usage of a drug.

Labelling matters because a drug may be approved only for certain variants of a disease, or for certain types of patients. Some drugs are approved as treatments for more than one disease or condition. The wider the approval, the bigger the market that the drug may be promoted to. Regulators may also require that drugs carry certain warnings.

The labelling does not directly restrict how doctor may prescribe a drug, but it does limit how a pharmaceutical company markets the drug, and marketing to prescribers is obviously a crucial (and expensive) part of selling a drug.

Clinical trials are carried out in three phases. The amount of information gathered and the number of people participating grows at each stage.

Pre-clinical testing

This precedes clinical testing and involves laboratory and animal testing of a drug. Pre-clinical trials need to provide enough information on safety to allow regulators to decide whether to permit clinical trials - whether the drug is safe enough to try on people.

Phase I clinical trials

These are the first trials with human beings. They typically involve a comparatively small number of people (a few tens of people) and are completed in a few months. The main purpose of phase I trials is to establish that a drug is safe enough to more on to phase II trials and to establish safe dosages. About 70% of drugs pass this phase.

Phase II clinical trials

Phase II trials involve more people, typically hundreds of people, and usually take longer (at least a few months, sometimes a year or two). The emphasis shifts from purely safety to looking for evidence of effectiveness, although safety continues to be studied. If no new safety issues emerge and the treatment appears effective in a significant proportion of patients then the drug can move to the next stage. About half the drugs that go into Phase II trials fail; only a third of the drugs that enter clinical testing will make it past this phase to go into Phase III.

Phase III clinical trials

Phase III trials involve hundreds or thousands of people and take at least a year, often several years. A phase III trial studies all aspects of safety, effectiveness and dosage. A control group of patients on standard medication (or none) is used to provide a yardstick for comparisons. This phase of clinical trials is clearly expensive but the majority of drugs that phase II also pass phase III. A drug entering Phase I typically has around a quarter to a third chance of making to to the end of Phase III.


Once a drug has passed through clinical trials it will take at least a few months (or even years), before it receives approval and can be marketed. About 20% of drugs that enter Phase I trials will eventually be marketed. After reaching the market the drug continues to be monitored for problems and risk of problems that could prompt withdrawal or narrowing of approval always remains, but slowly diminishes.