When you need to act quickly, saying that you need to act "in posthaste" is a great way to express urgency. However, if you want to mix things up a bit, there are several synonyms that you can use to convey the same sense of immediacy. Some alternative phrases to "in posthaste" include "stat," "immediately," "urgently," "ASAP," "pronto," "right away," "on the double," and "with all speed." Each of these phrases can help you communicate to others that there is no time to waste, and that action needs to be taken immediately. So, the next time you need to convey a sense of urgency, try mixing it up with one of these synonyms!