Hi canagirl
Depending on which type, steroids can have varying half lives (ie: how long they stay in the body) They also continue to act at a cellular level. eg: a steroid injection given in an inflamed joint can be expected to last 2-3 mths. These are given for temporary pain relief - they are not a cure. As you know steroids also have other not so beneficial side effects so they are often used as a last resort. Also depending on the type and cause of the PN they may not be suitable for idiopathic or other types.