Cortisone is really only useful if
1. a person needs to continue working a job that is aggravating the tendon that needs to be healed, and/or
2. there is significant pain such that starting physical therapy is hindered
In general, they do not fix the problem either long or short term. Almost all of the meta analyses say this.
The best thing for tendonitis -- aka -- acute is rest.
The best thing for tendinosis -- aka chronic -- is eccentric exercise and heat. Complete rest does nothing.
PRP, autologous blood injection, prolotherapy, and nitric oxide patches have all seen the most efficacious benefit otherwise.