If I decide to have vitamin C as a supplement, should I have an injection or tablet pills?

Vitamin c. Take in a pill form 500 mg daily. Injections of vitamin c can have reactions. There is a chewable form too as it can be associated with sins reflux symptoms because of ascorbic acid. You can get vit c over the counter from any grocery or pharmacy store.
Better as Powder. There is a supplement vit c powder from biotics called mixed ascorbate powder supplies 2767mg vit c, calcium and magnesium, make sure you do not have kidney issues. With vit c - best protocol is to use this as a vit c flush take 1 scoop every hour until it creates diarrhea take 1 scoop less then the total amount. Vit c in excess can create oxidative stress take lipoic acid to help this.