Depending on the state you can initiate a request for involuntary hospitalization for a patient. An evaluation from a physician and mental health professional will often be done to determine whether the patient meets the criteria for admission.
You can fill out a signed and notarized affidavit in some places expressing your concerns about the patient's behavior. You can also petition a presiding judge in some states to issue a court order for an involuntary hold if your loved one is exhibiting dangerous behaviors that put their safety or the safety of others at risk.