In AD&D, this is a staple item of any self-respecting adventurer's inventory; in NetHack, you never have one when you need one.

The blessed potion restores 8d4 hit points and cures blindness and sickness if applicable. If the restored hit points would put you over your maximum, you gain one hit point permanently.

The uncursed potion restores 6d4 hit points and cures blindness if applicable. As with the blessed potion, you gain 1 point to your maximum if the restored hit points would have exceeded it.

The cursed potion restores 4d4 hit points and doesn't cure jack s.*t. You do not gain anything extra if it would exceed your maximum. At least the potion doesn't cause damage instead, which is what the equivalent AD&D item would do.

Throwing the potion at a monster restores it to full hit points. Inhaling the vapors from a smashed potion of healing might give you 1 hit point if you're lucky.