Radioisotopes allow treatment and diagnosis of medical illnesses without the cost and trauma of open surgery (which carries risks like infection, accidents, and long recovery periods). The use of Tc-99m produces hot-spot images, which reveal early signs of disease and map how organs are functioning. The low energy gamma radiation it emits, the small dose in which it's injected and it a short half-life (6hours), minimises damage to tissues. The patient's total exposure to radiation is less than in an X-ray. So far, there have been no reported side effects.
But use of radiation on the human body can carry many side effects. For instance, the high-energy beta-emissions from Co-60 used in radiotherapy for cancer can destroy healthy cells, and cause hair-loss, skin burning, sweating, fever, pain, or decreases in the levels of different blood cells.
Gamma sterilisation (using Caesium-137) of bandages and surgical equipment has greatly reduced the spread of infection and made surgery safer.
Radiation irradiation of foods (using Carbon-14) increases shelf life and kill off any pathogens present in the food. This allows for safer foods, and lowers costs due to spoiling, but may also break up vitamins or nutrients in the food, decreasing its nutritional value.
Even minute amounts of radioactive material can be detected easily, which makes it ideal for use in tracing the movements of water, gases or even insects. In particular, the use of radio-tracers for leak detection in industry has made it possible to quickly locate and fix the leak with little disruption to the industrial process. However, the release of the radioisotope to the working/natural environment through the leak creates the danger of radiation exposure - for this reason, radioisotopes with short half-lives (like Na-24, half-life = 15hours) is used so radioactivity quickly disappears and the water/oil becomes safe to use...