Check out the MHRA website for info (for the public) on clinical trials,
There is a link to the expert report which was commissioned on the Northwick Park incident from this page also.
You should be able to determine some sort of risk level for what you are participating in by knowing at least if it is the first human study of a particular compound (as it was at Northwick park).
Another good indicator of risk and likelihood of adverse events or of other 'discomfort' is how much you are getting paid....
Before you participate you will be given a patient information sheet giving you all the available information on the toxicity and/or side affects of the drug which has been gathered from previous animal and human trials. Read it carefully and make sure you ask any questions about anything you are unsure about, and don't be fobbed off. Several cases in the past of investigators not even showing these to patients, and not telling them they are in a Clinical Trial at all, and forging their signatures on the informed consent form, sometimes because they knew that patients would be unlikely to give consent once they had read the potential outcomes on the information sheet.
I've never done a drug trial myself although I do work in Clinical Trials. Did have a lung biopsy once so they could test drugs on my tissue once, and got £400, but didn't have to take anything myself. That was nasty, would not want to repeat it.