Eleven can be obtained during these quest lines, twelve if the player character can pass a speech check convincing Jack Cabot that no serum was found, or thirteen if the player character sides with Lorenzo Cabot to the very end of the The Secret of Cabot House. Should the player character decide to free Lorenzo, they get access to an infinite supply of this item, with a few caveats: one may only receive one at a time and only when the Sole Survivor has none left, and the serum cannot be sold. Also one can not hoard the serum in storages, because Lorenzo will know if the Sole Survivor has a vial of it stored somewhere in the Commonwealth.
The item's description lists a Radiation effect of -36000, but this is misleading; it does not act like a RadAway. Rather, this number comes from removing 10 rads per second for 3600 seconds (60 minutes or one hour).
One potential use is for boosting carrying capacity while scavenging. The boost of +5 STR is significant, as the similar consumable Buffout only gives +2, and any chem mixed with it +3.
Another use is most helpful when the perk Ghoulish is acquired and fully upgraded. When standing in irradiated water without Radiation resistance, the player character will effectively take no new rads, but can gain up to 15 Hit Points per second.
The mysterious serum can be used to heal Andre Michaud in Far Harbor, but Lorenzo will no longer give the player any more if used in that way.
Five on a named raider, Lefty, who is stationed in Lorenzo's chamber.
One given by Lorenzo if the player character sides with him to the end of The Secret of Cabot House. This can be renewed if the player character has none left in their inventory. See details in the Characteristics section above.
The serum is derived from the blood of Lorenzo Cabot, father of Jack Cabot. It halts aging and gives the user incredible strength. It is through this serum that each family member has lived over 400 years. Though the in-game consumable is not addictive for the player character, the Cabot family are psychologically addicted. Jack's sister is the only one who realizes this, as she once went a whole month without a dose but ultimately couldn't stop.
It is mentioned by Jack, when talking about the raiders attacking Parsons, that the serum is diluted before the family uses it, and the serum one receives from the raiders and Lorenzo is the undiluted form. Jack explains that while the diluted serum halts aging, the undiluted serum also provides increased strength and damage resistance.
It is enormously helpful in Far Harbor, neutralizing the dangers of the fog on the Sole Survivor. The highly irradiated ocean will pull more than 10 rads per second, however.
Although Danse comments stating that you should give the serum to Brotherhood scribes during Emogene's Lover quest if you choose to give her the serum, there is no option to give it to the Brotherhood.