There are numerous synonyms for the phrase "is dangerous" that can be used to express the same meaning. Some of these synonyms include precarious, hazardous, unsafe, risky, threatening, vulnerable, treacherous, and menacing. These words can be used interchangeably depending on the context in which they are being used. For instance, precarious and hazardous are ideal synonyms to be used in situations where there is a possibility of someone or something falling, while risky and unsafe are better suited for situations where there is danger of harm or injury. Ultimately, the choice of synonym will depend on the specific situation and level of danger at hand.