It is produced by Mother Curie III by irradiating the recently distributed Aqua Pura. It provides +20 hit points, but also gives a high dose of +20 rads.
Effects of modified FEV
Drinking the water after choosing to infect it with the modified FEV will result in an additional player debuff effect and, if consumed in large enough quantities will cause death. The reason for this danger is the fact that the Lone Wanderer was actually born outside of a Vault and thus he or she is not immune against the modified FEV, which was designed to kill anyone who had prolonged exposure to the radiation and other mutational effects of the Wasteland.
The debuff effect changes the more water that is consumed. The follow shows the effects based on the amount of bottles ingested;
-1 Strength for 12 minutes
-2 Strength for 18 minutes
-2 Strength and -1 Intelligence for 24 minutes
Note: the bottle count stacks with Aqua Pura, resulting in death if 4 bottles of any mix of the two items are consumed (i.e. 2 holy water and 2 Aqua Pura).
Note: even if the effects of the debuff have worn off, the bottle count does not reset back to zero; resulting in death on the 4th bottle regardless of how much time has passed between each consumption.
After Rosa gives holy water to Micky, she will give one bottle to the player. She has one more in her inventory. An additional one can be stolen from the beggar at this time, by picking his pocket after Rosa gives him the bottle but before he drinks it.
On first meeting Brother Gerard he will hand the player a bottle. He has two to four more bottles in inventory, which can be pickpocketed.
If you drop all holy water from your inventory and speak to Gerard again after obtaining the first bottle, and he will hand over another bottle. This stops working after the first time the player drinks any, because that triggers the next step in Gerard's conversation script.
Xbox 360 Sometimes a message will report "holy water has worn off," similar to the message after drinking aqua pura. This may repeat at intervals indefinitely. There appears to be no ill effect, except annoyance.