The phrase "scares the pants off" is commonly used to describe something that causes extreme fear or anxiety. However, there are several synonyms that can be used to convey the same sentiment. One such phrase is "terrifies," which suggests a deep and intense fear. Another option is "petrifies," which implies a sense of being frozen in terror or shock. "Frightens" and "alarms" are also effective alternatives. Regardless of the word choice, the important thing is to accurately convey the level of fear that is being experienced.