The first step in testing a new treatment in humans. A Phase I study tests the safety, side effects, best dose, and timing of a new treatment. It may also test the best way to give a new treatment and how the treatment affects the body. The dose is usually increased a little at a time in order to find the highest dose that does not cause harmful side effects. Phase I clinical trials include only a small number of patients who have not been helped by other treatments. Sometimes they include healthy volunteers.