The importance of long-term follow up of participants in clinical trials

As good health is a lifetime issue, long-term follow up is an important part of evaluating any medical condition or treatment. This is well appreciated in epidemiologic studies where exposure to a harmful substance is often long-term, and its impact on health can appear many years after first occurrence. Examples include the classic study of Doll and Peto [ 1 ] on 34,439 male British doctors, which began in 1951 and last reported in 2004 after 50 years of follow up, in which lifelong cigarette smoking was shown to reduce life expectancy by an average of 10 years, but cessation at age 60, 50, 40 or 30 years reduced this by about 3, 6, 9, or almost the full 10 years, respectively. Other classic long-term epidemiologic studies have focussed on diet and alcohol consumption [ 2 , 3 ], hormone replacement therapy [ 4 ], or have been more general and studied a wider set of risk factors, often among healthcare professionals; for example, the ACS Cancer Prevention Study [ 5 ], the Harvard-based Nurses Health Studies [ 6 ] and the Health Professionals Follow Up Study [ 7 ].