Why are steroids dangerous?

Tim Cable, professor of Exercise Physiology at Liverpool John Moores University, replies: Anabolic steroids (AS) are used to aid recovery from heavy training and aid development of increased muscle mass. But there is no direct research evidence that says they do aid recovery and only limited data to suggest they increase muscle mass to a greater extent than training alone.
However, there is much evidence to show they can have a harmful effect, including an increased risk of liver damage, hepatitis and a long-term use link with liver cancer. They can also cause cardiac abnormalities (from irregular heart beat to heart attack) in otherwise fit, young and healthy men.
In addition, AS cause a marked decrease in high-density lipoproteins, the 'good' cholesterol that protects against heart disease. Plus they all have an androgenic effect, from testicular shrinkage to development of gynaemastia (breast tissue). They also increase levels of aggression and needle sharing may expose you to HIV.
In short, steroids are very, very bad.