What are side effects frequencies based on?

Question

I am one of these patients who reads everything that comes with their pills. Including, of course, the sometimes really long side effects list. Where I am, and from what I read online this is common, these side effects are sorted into several frequency categories (common/rare/very rare/sporadic, something like that). Yesterday, after years, I realized I had no idea where those frequencies come from. Do they come from scientific studies? From people participating in really large-scale trials? Or from doctors reporting back on what their patients report? From patients reporting it? But then, how are the frequencies determined? "Common", for example means something around 10 percent where I am, so you don't just need to know how many people report that side effect, you need to know how many people took the medication (and did so as instructed, and didn't take anything that interfered with it, etc.). So, **where do these frequencies and these lists of side effects come from?**

Answer 1

First and foremost, the frequencies, noted in the drug package insert, come from the clinical trials conducted by the pharmaceutical company that makes the drug. The FDA requires the sponsor to study the benefits and side effects of every drug that’s developed and approved for sale. These trials can be quite small (eg., 100 patients for some cancer types), or very large (eg., 10,000 patients for some cardiovascular diseases).

There is also a system that doctors (or anyone) can notify the FDA of a side effect of a drug that a patient takes. These reports are stored by the FDA and monitored for any unusual side effects occurring at a significant frequency. This FDA system of safety monitoring is particularly useful to capture infrequent but serious side effects that weren’t picked up in the smaller formal clinical trials. I should mention that the total number of prescriptions is tracked on a continuous basis and this provides the denominator for the incidence calculation.