“When evaluating data from VAERS, it is important to note that for any reported event, no cause-and-effect relationship has been established. Reports of all possible associations between vaccines and adverse events (possible side effects) are filed in VAERS. Therefore, VAERS collects data on any adverse event following vaccination, be it coincidental or truly caused by a vaccine. The report of an adverse event to VAERS is not documentation that a vaccine caused the event.”
https://vaers.hhs.gov/data/dataguide.html
Yes, I know that a VAERS report does not mean that a medication caused the event. Not all events are reported either. I am not sure how it can be determined with certainty that some reaction type of event was caused by a medication. Statistics can show probability with large enough sample sizes and close monitoring, but there is no way to do close monitoring on this scale.