Valkyr, also known as V, is a designer drug experimented with by Aesir Corporation. To most, it has the effect of any illegal drug out on the streets today, amped up by about a power of 10, but to some it empowers their senses, creating a super soldier, essentially. Valkyr was originally intended to be a performance-enhancing drug used by the military. The Aesir Corporation made Valkyr as a plan to create an army of soldiers impervious to fear or pain, but dropped the project after a 1% success rate,out of 100 trials. The drug then became illegal and went out on the black market for use as something of a high.
Valkyr produces strange hallucinations of flying Valkyrie of Norse Mythology, picking the spirits of the dead in battle. The difference between becoming an incoherent addict and a super soldier is all a matter of willpower. Physical and mental effects of taking Valkyr include nausea, hallucinations, and partial to total mental deterioration.
While the list is endless of those people who have gone crazy on Valkyr, those that were able to endure them with only "some" side-effects are few:
In Max Payne
V is routinely sold by the Italian mafia to junkies looking for a fix. Jack Lupino officially oversees the distribution of V, although he is nothing more than a middle man for the Don, Punchinello-who in turn is controlled by Nicole Horne, the antagonist who continued to research into the drugs and carry out experiments even though it was illegalised.
At the end of Part II, Max recieves an overdose of Valkyr by Horne. Max Payne then has lucid dreams and hallucinations in which he disturbingly finds out that he is in a graphic novel and inside a computer game. Max wakes from his nightmares in a pool of his own vomit, and experiences the after-effects of V as he makes his way to the next level, Cold Steel.
In Max Payne 2
V is briefly mentioned and seen in several places, but overall it has taken a backseat to the main plot. Max mentions that since the events of Max Payne 1, dealers and users of V has been on the decline. Vinnie Gognitti's men are overheard to have once peddled in V, confirming that they are part of the same group of dealers that Max fought in the first game.