If you have had or can get an MRI done that shows a herniated disc, then you can make a case for chronic pain. All you have to do is get your medical records, medical images, etc. and take it to a doctor who isn't ideologically opposed to the medicinal use of cannabis. The doctor's OK is the major hurdle you need to clear, unless the medical office itself is in trouble with the State (See: 11th State Consultations, Fall 2012). Then your RI Dept. of Health new patient form signed by you and the doc is mailed in, processed, and a decision will come back to you in the mail. Then you go to the State Offices and get your ID card printed.
You don't have to say anything (that "it works") to persuade anybody at the State to help you out, and they wouldn't anyway.