Hi Liz – you need to discuss this with your doctor since he/she gave you the prescription. Having said that, I have no theoretical explanation as to why a fatty acid that reduces inflammation would increase joint pain. I have an idea why joint pain was listed as a possible side effect, but that’s just a guess, so I won’t state that here. If you have fibro and found that EPA helps your mood, I will go out on a limb and state that your diet and lifestyle are probably causing a lot of inflammation. I strongly suggest you read up on anti-inflammatory diets like zone or paleo. Working with a dietitian (along with taking meds from your doctor) is likely to help a lot.