Laws vary w/states. States have different laws regarding involuntary admissions to psychiatric hospitals but in general if someone is a danger to themselves or others then an involuntary hospitalization can be pursued.
See below. Laws are state-specific. The general guidelines -- if you're in danger to yourself or others then you might be involuntarily admitted to a psychiatric hospital. In some states, if your behavior puts your in danger to yourself or others or mental illness prevents you to take care of yourself so as you're endangering yourself or others, you might be petitioned for an involuntary admission.