From Jonathan Cluett, M.D.,
Your Guide to Orthopedics.
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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

A. A cortisone shot starts to work very quickly once injected, although the time when you feel relief rom your symptoms can vary. Most people find that relief begins within 48 to 72 hours after the injection.

How does a cortisone injection help?
Cortisone shots work by decreasing inflammation. Once the inflammation subsides, pain relief follows. Cortisone starts to work immediately following the injection, and inflammation usually begins to subside within a few days. Depending on how quickly the inflammation subsides, timing of pain relief can vary from a few days to a few weeks.

If the inflammation is severe, or if the inflammation has been around a long time (chronic), the cortisone injection may take longer to take effect and may require more than one injection. Not every patient will respond to a cortisone injection, but the good news is that most people find this to be an excellent treatment for many common inflammatory conditions.

Can a cortisone injection make inflammation worse?
Some patients may have a reaction to the cortisone injection called a 'cortisone flare.' A cortisone flare is a condition where the injected cortisone crystallizes and can cause a brief period of pain, worse than before the shot. This usually lasts a day or two and is best treated by icing the injected area.

_Sources:

Cole BJ and Schumacher HR "Injectable Corticosteroids in Modern Practice" J. Am. Acad. Ortho. Surg., January/February 2005; 13: 37 - 46.

Fadale PD and Wiggins ME "Corticosteroid Injections: Their Use and Abuse" J. Am. Acad. Ortho. Surg., May 1994; 2: 133 - 140.