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Heavy G is a status effect in Mario & Luigi: Superstar Saga. Mario and Luigi are the only ones who can be affected with this status effect. They can receive this status by wearing certain Clothing, or by eating a Red Pepper. Cackletta's void attack can give the Mario Bros. Heavy G as well.
As its name suggests, the character affected falls faster on Jump attacks, and makes the Hammer and Hand Power attacks and defense slower, but lessens the input window. This makes it harder to attack or counterattack at the correct time. The status is indicated by a really big weight reading "1t" on the afflicted brother. In the remake, this only affects counter-attacking.