WebMD produce a useful patient information resource on supplements, over the counter medicines and prescription medicines, including interactions between them:
This link takes you to their Vitamins & Supplements Centre
This link takes you to their Drugs & Medications Centre
You can view information on interactions from the page of each individual supplement or drug but there is also a dedicated tool WebMDinteractionchecker
Other online drug interactions checkers can be found at:
PLEASE TAKE NOTE OF THE DISCLAIMERS ON ALL THESE SITES AND ALWAYS ENSURE YOU TAKE THE ADVICE OF A SUITABLY QUALIFIED MEDICAL PROFESSIONAL
Other useful resources include:
PainEDU Improving Pain Treatment Through Education have a number of helpful Interactive Tools relating to pain medications and pain therapies.