I just used a pill and received the message "Luck Down." I'm aware of Faith, which increases your chances of finding the Bible, and I suspect this is similar.
What is Luck?
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Luck is a stat that will increase the "level" of items that you find and your gambling output. For example, when you have the "Luck Foot" that Cain starts with, you cannot get "bad" pills such as bad trip, speed down, and other such bad effects. You can however get neutral and good pills like "Bad Gas", or Health up. I don't recall if Explosive Diarrhea is considered bad or neutral. You also get many more "victories" when playing blood banks, the shell game, and slot machines.
Luck stat can only be changed by taking pills 'luck up' or 'luck down'. It affects item drops after fighting in a room with mobs. It also affects the probability of tooth being fired when acquired the item 'tough love'. so if your luck stat is 9, you will have tooth shots all the time. In fact, having a low luck stat is better than having high luck stat. The lower luck you have, the more game will give you item drops. According to the code in isaac WTL 1.48, if you have -9 luck stat, rooms will always drop a brown chest.