Pig Juice turns you into a pig, giving you pig abilities. This is a permanent curse unless cured. Turning into a pig opens up pig abilities, but also causes NPCs to treat you differently. Some will even refuse to talk or trade with you.
It can be found on Pigs and Wild Pigs as a rare drop. It is also sold by various vendors.
The pig curse can be cured with the use of Un-Pig Juice.