Our body has already Glutathione, and if you lacking such, you have to take Glutathione from the market. Many believes that Gluta is good for cleansing the system before it will take effect to make your skin white. Some people will not accept the Glutathione in their body because they already have it and taking it will produced too much. That is why, other get good benefits from taking Glutathione, while are don't.
In terms injection of Glutathione, it mainly for skin whitening because it directly injected into your skin and doesn't need to go through to your stomach. Both have the same goal which skin whitening, but mainly different process. I heard some people vomiting blood while taking Glutathione capsule and maybe the reason you have to consume it when you have stomach empty, but rather take ut with meal.