I don't have a specific scientific reference (not a scientist sorry) but it's about excipients used in all tablets - they actually make up a surprisingly high percentage of the stuff you swallow. They are of course controlled by safety standards but it seems quite possible that some excipients may cause unintended side effects in some people. Allergies are cited as the most likely as the excipients may include something that the patient is allergic to but doesn't know they are taking - lactose for example. Or possibly might interact with other medication. As I indicated, I don't think excipients cause joint pain - that's the letrozole. The only way to know if there is any difference is to compare, over a reasonable period of time. But I'd want my oncologist to sanction that.