With an experimental treatment, there will be some risk. Not a great deal will be known about the treatment. You will be closely monitored and treatment will be stopped if it is thought there is any risk for you.
Remember – even in phase 1 or 2 trials, the treatment has been carefully researched in the laboratory before it is given to patients.
With phase 3 trials, more is known about the new treatment so there is less risk that there could be a harmful side effect.