How to kill theself in Nethack

First off, no matter the corpse, never eat unless you just made that corpse or you have a method of preservation (tinning, icebox). Rotten corpses cause food poisoning (unless you're a fungus or a ghoul), and it doesn't take very long for a corpse to rot. And there isn't a visual indication of rotting, either, so the safe bet is to never eat something you didn't just kill. Most undead corpses are automatically rotten, as well. (See below for a more technical explanation.)

If the monster has poison or acid attacks, or is made of poison or acid, then there's a very high chance that you'll be poisoned or take acid damage. Both of these can be fatal, the former even instantly so, but poison and acid resistance will protect against this. Note that most poisonous corpses will chance to give you poison resistance, so this can be something to risk if you have enough health. Other elemental effects like fire, cold, and electricity are traditionally safe.

If the monster causes you to be stunned, hallucinating, or confused by natural attacks (as opposed to casting a spell), then it is highly likely you will be tripping after you eat its corpse. Some of these are very obvious. Actually, most creatures with negative effects like petrification will also carry over that effect with their corpse.

While it won't directly kill you, the corpses of teleporting creatures like imps, nymphs, tengu, and leprechauns will often give you teleportitis. Note that tengu do have a chance of giving you teleport control, so it's a good idea to start eating those when you do get stuck with teleportitis.

Unless you are of the Caveman profession or the Orc race, you are not a cannibal. This means that eating a member of your own race will give you a penalty in the form of the "Aggravate Monster" intrinsic (which is as bad as it sounds) and a penalty to your Luck. This penalty will also show up for non-Caveman, non-Orcs when eating any domestic dogs and cats (the line of creatures you can get as a starting pet, generally). Horses are fine to eat.

For specific exceptions, off the top of my head: kobolds are always poisonous but never grant resistant. Bats cause stunning. Wraiths are safe to eat despite being undead. All adult Dragons are safe to eat, as well as most babies, but baby green dragons are poisonous unlike their adult counterpart. Lichen and Lizards do not rot over time and carry no negative effects that some other members of the same creature class might, so they make excellent backup food.

The rotting process is a little bit complicated and based on luck more than anything. Every 10 to 29 turns, a corpse gains a degree of rottenness. For a blessed corpse, 2 degrees are subtracted, while for a cursed one 2 degrees are added.

When it reaches a rottenness of 4, it is considered old. When you eat an old corpse, there's a 20% chance you will lose 1-8 hp, and a 1/7 chance of it being rotten (as in rotten food, not deadly.) When it reaches a rottenness of 6, it will give you fatal food poisoning, killing you in 11-19 turns.

So, put more simply, a corpse is definitely safe to eat in all cases for 32 turns, and it can cause food poisoning after a minimum of 48. Everything beyond that is just random chance.

Undead corpses start out with an age of 100 turns, which is why they are almost always already rotten.

Attributions for information provided within comments to:

• WillfulWizard for reminding about teleporting monsters.
• ire_and_curses for corrections on non-rotting lizards and poisonous baby green dragons.
• Kaestur Hakarl, who investigated and discovered the safe measures for cannibalism.

There are two parts to the hunger problem in NetHack: gaining nutrition and reducing nutrition loss.

Under normal conditions, each turn, you lose one point of nutrition. You start with 900 nutrition, and start feeling hungry when you drop below 150. At 0, you'll faint. Starving to death happens at a negative value based on your constitution.

Nethack is a very complex game, so there are exceptions to most of the guidelines below, and other, more obscure ways of sustaining yourself, but these general principles are good guidelines.

Gaining nutrition

• Comestibles. The most straightforward way to gain nutrition is to eat comestibles (labeled as % - food, meat and corpses). The amount of nutrition you get depends on what you eat. Early on in the game is often the riskiest time; you may not start with much food. At this stage, it's normal to eat as many fresh, safe corpses as you can get your hands on. If you make it that far, there is guaranteed food in the Sokoban levels, which are accessed somewhere between levels 6-10.

• Praying. Praying while weak will restore your nutrition, but safe use depends on your current prayer timeout. Also, using up your favour with your god could backfire if you have a sudden unforseen need (e.g. you have gained a fatal illness like food poisoning, or are being turned to stone).

• Convert boulders to food. As theist points out, you can convert boulders to huge chunks of meat if you have the stone to flesh spell. These are hugely nutritious (2000 points).

Reducing nutrition loss

• Wear a ring of slow digestion. This is a great item, if you can find one. It changes your rate of nutrition consumption to 5% of your normal rate (giving you twenty times as long to find food). As Grace Note points out in the comment below, this is because you are only paying the nutrition cost of the ring - your standard rate of consumption is halted.

• Take off your rings. In general, you lose one nutrition every twenty turns for each ring or amulet you wear. Rings of regeneration are particularly pernicious; if you're wearing one, you'll lose a point every other turn.

• Drop your stuff. If you're Stressed or heavier, you lose a point every other turn.

• Don't cast spells. Spellcasting makes you hungry (unless you're a wizard with high Intelligence).