The way that I had it explained to me by my podiatrist is that cortisone works as a massive local anti-inflammatory agent by literally degenerating the tissues that are causing the inflammation. This could be a fluid filled bursars sac, etc. It takes approximately 3-4 days for typical types of cortisone to fully absorb into the area around the injection site. At this point, the cortisone has done all that it can and things usually feel better. If the site has no reason to get inflamed again, all is well. If the injection site still has an underlying problem that causes new inflammation, you're pretty much back where you started. Depending on where in the body the inflammation is located, the doctor may recommend further injections. Typically, they only do knees once, since the tissues around the knee are relatively fragile (some doctors won't even do one in the knee). For the Plantar Fascia, there are differing opinions as well but in general the thought is that more than one can be done (check with your doctor on this one). You must keep in mind that you are further degenerating bodily tissue for each injection that you receive. There have been cases where cortisone injections have caused Plantar Fascia ruptures, although supposedly this is uncommon. From what I've heard, this can happen after only one injection, or could happen after several. The up side to a rupture is that the Plantar Fascia has now been released and you've saved yourself surgery. The down side is that it's very painful and you could have the secondary problems associated with a Fascia release (nerve problems, fallen arch, etc.) You need to carefully consult with your doctor on this one, even get a second or third opinion. Definitely keep track of the number of injections that you've had if you have different doctors do them. Also, be sure that the injection site is where the pain is occuring, otherwise it isn't of much use. Some doctors mix the cortisone with novacaine (sp?) and have you stand up approx 1 minute after the injection. If you can only feel numbness and no pain where you felt pain before the injection, the doctor got the proper spot. For one of my injection trials, the doctor missed the first time (due to my directing him to the wrong spot) and had to inject me again right away in the proper spot. Bottom line, ask a lot of questions before going through with it. Sometimes, people need something to get them 'over the hump' when dealing with PF. Good Luck