Cotisone is an anti-inflammatory. You have typical features of impingement in your history. This is where the tendon gets pinched between the upper end of the arm bone and the undersurface of the acromion (point of shoulder). Inflammation in this area leads to pain and the cortisone relieves this. Cortisone takes 10 -14 days to work. There is often discomfort the day after the injection.
Contary to other opinion on this forum it is atreatment and diagnostic tool for impingement. It also helps with cuff tendinosis and calcific tendinitis. It will not help with weakness if there is a cuff tear
The injection may last for ever or your symptoms may return. The fact that the injection has helped is importnet for the Dr treating you. The important thing is to avoid multiple steroid injections in a short space of time (<6 week intervals) as this can weaken the cuff.
If your symptoms return surgery is an option but full recovery takes one year after this.
Good luck
PS -ask your DR re return to golf but I would expect 6 weeks post injection would be reasonable and safe
Thanks Julian. Still no pain and I'm golfing. Lifting very light weights right now but want to go heavier soon. I'm scared the shot will wear off and the pain will come back. Time will tell. Thanks again, I thought I was forgotten.