Can you get collagen injections in your breasts?

Yes. Collagen injections typically last no longer than a couple months, and there is a risk of allergic reaction occurring. Additionally, to make much of a difference in breast size, and very large volume of collagen would have to be injected, and this would be excessively costly.
Not really! Collagen comes in very small quantities and only lasts for 3-4 months. It would not provide any visible change that would last any real length of time. There are better choices for breast enhancement.