Can you bring the rakghoul serum to both the doctor and Zax, getting the reward from both?

[Paul Bunyom]

Or if you give it to the doctor before giving it to Zax, it's basically worthless to sell at a high value?

[[L]Sanders]

Once you give it to one of them, it leaves your inventory, and updates the quest log. You can't give it to both.

[jcansler520]

What Sanders said. Technically you can buy another vial of it from the doctor after giving it, but the whole point of choosing one or the other, and the price you get for it, is because that the doctor makes it a widely available thing everyone can get, while Zax makes it useable only to a select few. Hence why the guy tells you how you "blew it" when you give the vial to the doctor. It no longer has the same price value then. If you give it to Zax, you can't get the vial back, so either way, it won't work.