The development of preventive therapy for cancer is still in its infancy, and much can be learned from cardiovascular medicine, where it has now become firmly established. This is due to the existence of agents with clearly proven efficacy and minimal side-effects, such as the statins for cholesterol-lowering and the antihypertensive agents to control blood pressure. For cancer, only vaccination against the human papilloma virus to prevent cervix cancer and hepatitis B to prevent liver cancer, aspirin for gastrointestinal cancers and anti-oestrogen agents for breast cancer have been clearly established as effective. 

There is a clear contrast between two different agents for preventive therapy – aspirin for gastrointestinal cancers – notably bowel, stomach and oesophageal (gullet), and endocrine therapy for breast cancer prevention. For aspirin the benefits are seen for a range of different cancers, making identification of high risk individuals difficult. For most individuals there are no side effects and the potential benefits outweigh the potential harms, but it is highly desirable to identify the small subset most likely to suffer side effects (gastrointestinal bleeding) and to not offer aspirin to them.  In the case of endocrine agents such as tamoxifen or the aromatase inhibitors, the benefit is expected only for breast cancer. Development of highly predictive risk assessment models is important for identifying women most likely to benefit from these drugs.