"You turn into a newt! Your armor falls around you! You find you must drop your weapon! --more--"
"Demogorgon hits! Demogorgon hits! Demogorgon stings! --more--"
"You die...--more--"
"Do you want your possessions identified? [ynq] (y)"

Don't drink this in a combat situation unless you are truly desperate, as noted above.

This one of the only NetHack potions I can think of where blessed/cursed status is completely irrelevant. You quaff, you polymorph. You throw it at something else, they polymorph. Remember that polymorphing occasionally causes a system shock that deals you 30 damage immediately.

This potion is ineffective if you possess the unchanging intrinsic.

Starting with a potion of polymorph is something really great. Throw this potion to your pet and it is likely to change to a more useful form than a kitten/little dog. Last time this situation happened, my pet changed to a blue dragon which killed everything in sight, even shopkeepers.

If you don't know the potion of polymorph at the start of the game (which is the case 99.5% of the time), I would recommend you to take off your cloak and armor before you drink a costly potion because if you ever hit this potion, you might break your armor and cloak permanently...