Shoulder injection. Man receiving an injection in the shoulder as part of treatment for arthritis or tendinitis. The syringe contains a corticosteroid anti-inflammatory drug which will alleviate the symptoms rather than cure the condition. Arthritis is a painful, swollen and stiff joint due to one of a number of causes. Tendinitis is an inflammation of a tendon due to injury; tendinitis in a shoulder is sometimes called painful arc syndrome. Corticosteroid drugs work by supplementing natural hormones, reducing the production of inflammation-causing prostaglandin hormones. Corticosteroids also suppress the immune system.