It's the Mental Damage

Drink Like a Demon has two problems. The first is that it's an uneven trade; you're losing 4 points of ability score and only getting 2 back. This does a few things to your Drunken Master that are bad:

• Lowers AC
• Lowers vital Wisdom-based skills (such as Spot and Listen)
• Lowers your Will saves
• Lowers your save DCs for your special abilities
• Makes you more vulnerable to monsters and spells that deal mental ability damage

While you can run Fist of the Forest to mitigate the AC problem, all you ever end up doing is breaking even - and that's if you take the Con, and not the Strength. If you want a melee benefit out of Drink Like a Demon you've gotta burn AC point after AC point while simultaneously making yourself increasingly weak to save-or-suck and save-or-lose spells like charm person, hold person, and anything else that requires a Will save. The end result is that you turn yourself into a glass cannon that, at any moment, could be pointed at your party by a monster.

Sadly, not even the common "fix" of going Kung-Fu Genius works; Drink Like a Demon lowers your Intelligence.

Additionally, as Tridus mentioned in the comments, each drink you take is burning a move action, meaning that every round you take a drink is a round you cannot move, use Flurry of Blows, or make a full attack. Essentially, drinking makes you worse at doing your job (delivering damage to targets of opportunity) while also making you more vulnerable.

First, it must be said: monk is a weak class. The best number of monk levels you can have is zero, and unarmed swordsage would be literally strictly-superior. I am assuming, per the question, that the first two monk levels are set in stone, but if they are not, I would strongly consider alternatives.

For a quick run-down...

Monk 1 is a fairly solid level

You get two bonus feats (Improved Unarmed Strike and one other), great saves, and Wis-to-AC. The HP, BAB, skills, and proficiencies are all mediocre, but oh well, they’re not exactly atrocious either. Flurry of Blows... well, it’s optional, so you can always opt to not use it.

Two bonus feats in one level is worth a fair bit, so cool.

Monk 2 is... OK enough

Evasion is pretty good, another level of all-good saves is solid, and hey, yet another bonus feat. You can do better but it’s not awful. If you aren’t using sane multiclassing rules for BAB (read: fractional), the BAB’s also as good as you’re gonna get. So yay.

Monk 3 and 4 are just bad

Still Mind is a small bonus to saves against only a single school of magic. It doesn’t even cover all mind-affecting things. There’s a reasonably good chance that you will forget you have this bonus when the time comes to roll a save it works on.

Fast Movement is a little better; you’ll probably remember you have it, anyway. But it’s still not great; it is not something that’s going to be particularly relevant most days, since you typically want to stay with your party and rarely are going to need more than 30 but less than 45 feet of movement in combat.

Ki Strike is OK enough, except you need magic bonuses to your unarmed strikes anyway. So you are going to have an item that obviates this class feature. The better option is necklace of natural attacks^SS; a +1 amulet of mighty fists is ridiculously overpriced, but will cover your need for magic damage.

Slow Fall is just insulting.

The unarmed strike damage improvement works out to an average of +1 damage. You can do better.

Alternatives

So, what to take instead of those levels of monk?

Cleric

Cleric is almost-certainly the best single-level dip in the game. Won’t improve your HP or BAB, but it will give you fairly-good saves, and more importantly a couple of spells and the right to pick a couple of Domains. This is a big deal.

The best choice for you is to take the Travel Domain, and then swap it for Travel Devotion.^CC This just about instantly solves your mobility issues, as it allows swift-action movement for a whole minute each day, with additional minutes gained by burning Turn Undead uses. A Cha of 12 is sufficient to get 3 minutes of this every day. If you have low Charisma, or just want to make sure you always have enough Turn Undead uses, you could just take the Undeath Domain to get Extra Turning.

Other solid options include the fantastic Time Domain, for Improved Initiative, a solid feat in its own right that is also often required for things. Along those lines, the Darkness or Shadow Domains get Blind-Fight, which is kind of mediocre but is another common requirement. War gives Weapon Focus, and deities who favor unarmed strikes exist, but I would only do that if I were eyeing some things that require Weapon Focus, as the feat is quite poor.

For non-feat options, you could do a whole lot worse than the Magic Domain; a single level in cleric paired with the Magic Domain means you get to use every single cleric and sorcerer/wizard wand in the game, with no Use Magic Device check. That is really nice, particularly on a character that isn’t going to get UMD in-class or have high Charisma.

Other meaningful Devotions^CC include Animal (flight! limited, sure, but still!), Law (attack bonuses; you need them), and Knowledge (you probably won’t have great Int, but it’s a minimum of +1 to attack and damage no matter what you roll, and it’s not super-hard to get +2 or +3). Knowledge Devotion is so good that it’s worth considering being a cloistered cleric^UA just to get it. Under fractional BAB, Monk 2/Cloistered Cleric 1 has +2.0 BAB, making it a good time to start taking +1.0 BAB classes.

Paladin

Two levels of paladin gets you Divine Grace, which you shouldn’t care about in the least since you should have minimal Charisma, but the Serenity feat^DC allows you to switch Divine Grace to Wisdom, and then all of a sudden it matters a whole lot. It also swaps other paladin class features; Smite Evil 1/day isn’t impressive, but you’ll have it and it will use Wisdom instead of Charisma.

Really, I only recommend this route if you’re going high-Wisdom, which your question indicates you are not. If you change that, I also recommend Intuitive Attack^BoED as that will reduce your need for Strength; it functions like Weapon Finesse, but for Wisdom instead of Dexterity and for simple weapons (including unarmed strikes) rather than light ones.

And the problem with this is that you already need two feats (Great Fortitude and Power Attack) by 5th level in order to qualify for fist of the forest, and paladin wants another two feats. Non-humans only get two feats by 5th unless they have flaws,^UA which means you can’t actually take Intuitive Attack and Serenity until later. Being human helps (you can get one of them) and flaws eliminate the problem, but if those aren’t options, those paladin levels don’t do a lot for you until 6th or even 9th.

Psychic Warrior^XPH

Another ¾ BAB class, sadly, but this one is a powerhouse. The first two levels have bonus feats from the fighter list, or any Psionic feat, so that’s a ton of options. More importantly, you get some powers, including the excellent expansion, and if you really want to, you can take Monastic Training^EbCS (psychic warrior) and Tashalatora^SoS to have psychic warrior progress your various monk class features. In fact, since neither feat requires levels in monk, you could replace monk entirely with this class.

Ranger

Fist of the Forest requires a fair few skills, and they’re kind of ranger-y. Ranger has full BAB and 6+Int skills. It’s boring, but it’s thematic and it will help you out. I wouldn’t bother with more than one level, though. I’d try to avoid even that if I could.

Warblade^ToB

OK, now we’re getting somewhere. Warblade has d12 HD, 4+Int skills, and a few maneuvers – which do excellent things for your mobility and versatility. Diamond Mind is excellent, as it is based on Concentration (which, obviously, uses Constitution), and Stone Dragon can do great things for that whole survivability thing. In particular, knowledge of a Strone Dragon maneuver can let you take Stone Power^ToB instead of Power Attack, trading attack penalties for temp HP every round. That can seriously extend your HP, as you keep regaining temp HP.

Personal Suggestion

Dragon of the Stony Path

Assuming fractional BAB,

Monk 2/Warblade 1/Cloistered Cleric 1/Warblade +1/Fist of the Forest

Cloistered Cleric should definitely get Travel Devotion and Knowledge Devotion. For the third Domain, Magic and Time are strong contenders, as is Law Devotion. But Undeath Domain is probably the best choice, not because it’s particularly amazing, but because Extra Turning means you can dump Charisma and get 4 uses out of Travel Devotion in a day. Travel Devotion is fantastic.

Warblade is more interesting. That first level, I’d probably take stone bones, considering your goals, and sapphire nightmare blade to give your Constitution some offensive use. Douse the flames or leading the attack are my picks for the third maneuver, but either works. The stance is a toss-up too, but I favor bolstering voice or leading the charge.

Then, because you have a Stone Dragon maneuver, you can pick up Stone Power at 3rd level. This meets fist of the forest’s Power Attack requirement, and is decidedly more survival-oriented. It’s also just better when you aren’t wielding a two-handed weapon.

For the second warblade level, your Initiator Level is 3 or more, so you can take a 2nd-level maneuver. Plenty are good, but mountain hammer is more-or-less definitively the one you want to take. It’s good in combat, and great out of combat. Seriously, get mountain hammer.

Worth noting, if you have the flexibility to change the order of your classes, 1st level has some special features (maximized HD, quadrupled skill points) that monk does not make good usage of. Either of the other classes is better; warblade for HP or cleric for skills.

Zen Knight

Monk 2/Paladin 2/Warblade 1/Fist of the Forest

Using Serenity and Intuitive Attack to add Wisdom to all saving throws (twice to Will!) and to your attack rolls, as well as your AC of course. Warblade either at 3rd, so you can take Stone Power, or at 5th, so you can take mountain hammer. If retraining is allowed so you can swap out Power Attack for Stone Power once you have your warblade level, it’s far better at 5th. If not... ugh, that’s an unpleasant choice.

Remember the problems brought up in the paladin section about actually taking Intuitive Attack and Serenity while qualifying for fist of the forest.

But this basically isn’t worth it unless you are going all-in on your Wisdom score. Paladin does not add very much – not nearly as much as cleric, swordsage, or warblade – other than the opportunity to use Wisdom for almost everything you do.

Note that unarmed swordsage is so much better than monk here. It’s always true, but especially so in this case. Swordsages get Wis-to-AC, but can still wear light armor. It would also get you a bunch of maneuvers, potentially helping with the mobility issues you have because you don’t have Travel Devotion.

On which note, Monk 1/Cloistered Cleric 1 (assuming fractional BAB) is far better than Monk 2, and maybe even than Swordsage 2: you get Travel Devotion.

Monk 1/Paladin 4/Fist of the Forest

Only worth noting because Serenity means that Paladin 4’s Turn Undead is 3+Wis uses per day instead of 3+Cha uses. Then you can take Travel Devotion as a regular feat. Don’t really think it’s worth it, myself, unless you cannot use fractional BAB. Particularly since you already have issues getting all the feats together.

SAD Wisdom-sensei

Unarmed Swordsage 1/Paladin 2/Unarmed Swordsage +3/Fist of the Forest

Note that this build qualifies to take fist of the forest at 6, but doesn’t because Divine Grace and Insightful Strikes are yummy.

Intuitive Attack and Serenity are assumed. Again, accomplishing that as a non-human without flaws is problematic. At least here you can delay Great Fortitude until 6th, that helps. Also, swordsages really want Adaptive Style^ToB...

This build gets Wisdom to AC (even while wearing light armor), all saves (twice to Will!), attack (with Intuitive Attack), and damage (when using a strike from the discipline chosen for Discipline Focus). Stone Dragon is a decent choice for Discipline Focus, as it will give you Weapon Focus (unarmed strike) as well as be a discipline you want to use often, but note that Stone Dragon has the unfortunate limitation of only being usable while standing on the ground. This becomes a serious problem at higher levels, when it becomes so crucial to get flying in order to engage foes.

Sources

• ^BoED – Book of Exalted Deeds
• ^CC – Complete Champion
• ^DC – Dragon Compendium
• ^EbCS – Eberron Campaign Setting
• ^SoS – Secrets of Sarlona
• ^SS – Savage Species
• ^ToB – Tome of Battle
• ^UA – Unearthed Arcana
• ^XPH – Expanded Psionics Handbook