last updated 4/4/2024
There is just a single case report of fish oil causing hypomania, as far as I can find searching for others in Pub Med. Lack of other reports suggests this one case was some sort of fluke. Here’s a potential reason for that fluke: the fish oil in this case contained more DHA than EPA.
Several large analyses (example) suggest that EPA is the key ingredient in fish oil for mood. Or at least EPA should be the dominant omega-3 in each pill, relative to DHA.
So for now I think it’s fair to conclude that fish oil (with EPA > DHA) does not carry a risk of causing hypomania. This is contrary to several summaries (example) which state this risk but do not cite references.