The player character must give Senior Scribe Neriah 20+ blood samples in the unmarked quest Blood Bank, and then talk to her again. She will give the player character 6 X-111 compounds and will inform them that if they want more they need to bring her more viable blood samples.
Six are given by Neriah after giving her more than 20 viable blood samples. Neriah will take all viable blood samples from the Sole Survivor's inventory and pay 50 caps each. Speaking to her again and choosing X-111 will award six X-111 compound if 20+ viable blood samples have been turned in since the last time. To maximize the amount of X-111, drop or store all but 20 viable blood samples before speaking to Neriah.
The Sole Survivor with the Medic perk at rank 3 will find this less useful as X-111 compound weighs three times as much as RadAway, which in Medic rank 3 reduces Rads by 800.
Even without Medic, the X-111 compound is less efficient than RadAway as 0.3 weight of RadAway removes 900 rads, compared to the 700 of one X-111 compound.
Further, the rad-removal comes in a larger dose, which forces the player to wait longer before using the compound if they don't want to waste it.
On survival difficulty X-111 only removes 60 Rads but it removes them instantly while RadAway removes them over time. Unlike RadAway, however, consuming X-111 does not induce Fatigue or temporarily increase vulnerability to illness.
Scribe Neriah states that it seemed appropriate to name the compound 'X-111'. This is because the Sole Survivor entered the post-War world from Vault 111.