We've all encountered the maniac at one time or another; with a seemingly unbelievable disregard for money and inexplicable tolerance for risk the maniac can at times cow the less observant players but is ripe for exploitation by an astute student of the game. It is important to understand that playing against a player of this caliber is bound to increase your variance but it will also increase your win rate and is always a beautiful opportunity.
It is important to play forcefully against them, although slow playing can work well versus some of the fold to a raise types. As is standard against any extremely aggressive player you will want to pick your spots but you will also want to go alot further with marginal hands like TPTK and an overpair.

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One classic method of exploiting the maniac is limp/re raising with medium strength hands like AQ or 1010; this can be put to extremely profitable use at a very loose table. This is ideal if the maniac is immediately to your left but can work from a variety of positions. Preflop you will generally want to re raise big or fold against a maniacs raise unless you have a strong handle on how they will play after the flop.
It's good to re raise strong with big pairs because maniacs are apt to call your raise, allowing you to get a substantial amount of money in before the flop when you have a strong advantage. Of course the mistakes the maniac makes compound against him, forcing him into tougher and tougher post flop situations.
The key to extracting the maximum from a maniac is simple; stick to your tight, solid preflop play and then punish him when you have a hand with large raises and re raises. Emotionally prepare yourself for larger swings and rest easy knowing that playing a maniac at Texas Hold'em is probably one of the easiest ways in the world to make money, if you have the grit and determination to pull through.