There is no scientifically proven limit for cortisone injections, however as a general rule, three injections into the same body part are permitted over a twelve month period. Injections more frequent than this are felt to place the injected tissue at risk of softening/weakening, which may be an issue in a joint for example, as this may accelerate arthritis. Also, if you have failed to respond to a series of three injections, then it is probably time your condition was reassessed to find out if the diagnosis correct. Has your condition worsened and are other forms of treatment, such as surgery, more appropriate?
If you do require more than three injections in a year, then the risk of the injection must be carefully balanced against the benefits of pain relief.