A cortisone shot typically starts providing relief from symptoms within 48 to 72 hours, according to About.com. Cortisone shots relieve pain by reducing inflammation. Inflammation begins to subside immediately after the shot although significant pain relief can take a few days to a few weeks.
Cortisone shots sometimes take longer to provide relief from severe or long-lasting pain, notes About.com. Relieving severe pain sometimes requires more than one shot. A minority of people do not get relief from cortisone injections. In some cases, patients react negatively to a cortisone shot with a "cortisone flare," in which the cortisone temporarily makes pain worse.