Being on full alert is when someone is vigilant, watchful, and fully prepared for any potential danger or threat. Synonyms for this phrase can vary depending on the situation and context. Some alternatives to "on full alert" include being on high alert, being fully attentive, being on guard, being extra cautious, being on red alert, being on heightened awareness, being on standby, being ready for action, being on the lookout, and being on the edge. These synonyms can be used interchangeably to describe someone who is alert and prepared to respond quickly and decisively in any given situation.