This move effect may be in need of research. Reason:Does the Belly Drum bug apply in Stadium 2? You can discuss this on the talk page.
Belly Drum deducts half of the user's maximum HP (rounded down) from its current HP and, in return, it maximizes the user's Attack stat by raising it to +6 stages, even if the user's temporary Attack bonus stages were below 0 prior to using Belly Drum.
Belly Drum fails if the user's current HP is less than half its maximum, or if the user's Attack is already at +6 (even if the user has Contrary).
If the user has Contrary, its Attack will be lowered to -6 stages (although the message will still say that the user maximized its Attack); even if the user's Attack is already at -6 stages, Belly Drum will deduct HP.
In Generation II only, if the Pokémon is under 50% HP when using the move, it will still have its Attack raised 2 stages due to a bug.
In the Pokémon Mystery Dungeon series, Belly Drum has the same Attack-maximizing effect but empties the user's Belly (to only 1 point) instead of reducing the user's HP. It has no effect if the user's Belly is at one or is empty.
In Super Mystery Dungeon, it instead reduces the user's Belly by 50. If it is used when the user's Belly is 50 or less, it fails.
While Shell Smash causes a net total of eight stat changes, Belly Drum can cause more stat changes if the user has experienced at least a -3 stat change to its Attack stat, reaching twelve stat changes if the user has a -6 Attack stat change.