Hi,
The title says “adverse event detection” but based on your approach it looks like you want to try “closed book question answering”.
If “adverse event detection” is all you need then you might want to train a simple encoder only model like pubmed_bert released by Microsoft on entity recognition task.
But in case you want to identify answers to other questions including “adverse events” using a QA approach you might have to first pretrain a seq2seq model on biomedical domain which should have a broad vocab and later finetune it for QA task.
I think closed-book QA is already a challenging task and if we are using a model which is not pretrained on biomedical domain, we might not get desired results.
Still there is no harm in trying, just sharing details based on my experience. Ignore if not useful 
Cheers !!