How effective really is steroid injection in the treatment of alopecia areata as a coadjuvant?

It is not a considered a cure but it is the only treatment available.  If you are willing to go through with the pain of the injection and the cost of the treatment it may or may not work

In my opinion, intralesional injections with triamcinolone  are helpful for most cases of localized alopecia areata. Your question is regarding adjuvant therapy. For this reason, I'm also going to suggest that you may want to try excimer laser as well as platelet rich plasma together with the intralesional injections to improve the results. You also need to be careful with receiving intralesional injections for a prolonged period of time. There are changes of steroid atrophy as well as changes in calcium metabolism. Hope this helps. Good luck.

If steroid injections (triamcinolone acetonide) are going to do anything, results are seen in 4-6 weeksSteroid injections work very well for patchy alopecia areata, and work less well for alopecia totalis and not very well for individuals with alopecia universalis.You can take one set of injections every 4-12. You require a dermatologist to assess the safety of frequency of injections and the concentration of the triamcinolone (generally 2.5 to 5 mg/mL)If you don't see results after 1-2 visits, I advise stopping. 

This works sometimes and sometimes it does not work. This is a difficult condition to treat so it should be managed by a dermatologist who specializes in this disease.