What does a steroid shot do for pharyngitis ? does it kill the cause ?

Anti-inflammatory;no: Presumably you're referring to a corticosteroid shot rather than an anabolic steroid such as testosterone. Various forms of Prednisone (steroid) can be given as pill or shot, even as cream, all acting as anti-inflammatory to cut down on swelling. However, it doesn't "kill the cause" which is most likely due to either a virus or bacteria. If the latter, need antibiotics. If former, just time.