Recent significant trauma such as a fall from a height, motor vehicle accident, or similar incident 
Recent mild trauma in those older than 50 years of age: A fall down a few steps or slipping and landing on the buttocks may be considered mild trauma. 
History of prolonged steroid use: People with asthma, COPD, and rheumatologic disorders are frequently on this type of medication. 
Anyone with a history of osteoporosis: An elderly woman with a history of a hip fracture, for example, would be considered high risk. 
Any person older than 70 years of age: There is an increased incidence of cancer, infections, and abdominal causes of the pain. 
Prior history of cancer 
History of a recent infection 
Temperature over 100 F 
IV drug use: Such behavior markedly increases risk of an infectious cause. 
Low back pain worse at rest: This is thought to be associated with an infectious or malignant cause of pain. 
Unexplained weight loss 
The presence of any of the red flags would justify a visit to a hospital's emergency department, particularly if your family doctor is unable to evaluate you within the next 24 hours.
The presence of any acute nerve dysfunction should also prompt an immediate visit. These would include the inability to walk or inability to raise or lower your foot at the ankle. Also included would be the inability to raise the big toe upward or walk on your heels or stand on your toes. These might indicate an acute nerve injury or compression. Under certain circumstances, this may be an acute neurosurgical emergency. 
Loss of bowel or bladder control, including difficulty starting or stopping a stream of urine or incontinence, can be a sign of an acute emergency and requires urgent evaluation in an emergency department. 
If you cannot manage the pain using the medicine you are currently prescribed, this may be an indication to go to an emergency department. Generally, this problem is best addressed with the doctor writing the prescription who is overseeing your care for this problem.