There is no guarantee of 100% safety by taking part in a clinical trial, because the purpose of clinical trial is to establish safety and whether the treatment works for a particular condition.
A researcher would never ask you to take part if they thought the risks outweighed the benefits, nor would an ethics committee approve the trial.
Ethics committee review the safety data to make sure the expected risks don‟t outweigh the expected benefits and that the trial is scientific and appropriately conducted. During the trial the researchers will pay extra special attention to you make sure your health is being looked after.
Go back to the clinical trials basics section for more information to help you work out the level the risk of a clinical trial and whether it is right for you.