Recent research shows that the PAs found in borage (and other PA-containing plants) can be taken up by other plants when grown in close proximity or when comfrey is used in the garden as mulch or fertilizer.
August 2021 Safety Update: Borage contains pyrrolizidine alkaloids (PAs), which can be harmful to the liver over time when ingested internally. Recent research shows that the PAs found in borage (and other PA-containing plants) can be taken up by other plants when grown in close proximity or when comfrey is used in the garden as mulch or fertilizer. In light of this, we are recommending that borage should not be interplanted with herbs or food plants in the garden that will be ingested or used as mulch or fertilizer to err on the side of caution. However, mature compost that includes borage does not seem to contain PAs once it has been fully composted according to this study.