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Dragonball  Z/GT: Senzu Bean Guide

Usage #1 – 1 bean (Red Ribbon Army)
When Goku climbs Korin Tower in the Red Ribbon saga, he is at one point extremely hungry from not eating for about 2 days. Korin tosses him a bean that is said to single-handedly satisfy one's hunger. This is the first appearance of the senzu. 

Usage #2 – 1 bean (Piccolo Daimao)
At Korin's Tower, Goku uses another bean to heal himself from the fight with Piccolo Daimao. This is when the true advantage of the senzu bean is first realized in the series.

Usage #3 – 2 beans (23rd Budokai)
1. After the Goku vs. Piccolo fight, the final one of the 23rd Budokai, Yajirobe makes his first of many "Senzu deliveries". Goku uses a bean to heal from the intense fight.
2. Goku, for no apparent reason at all, gives Piccolo one of Yajirobe's senzu. He later says that it was an act of pity, and "no matter how strong Piccolo becomes, I will always be one step ahead."

Usage #4 – 2 beans (Saiyan saga)
1. After Goku leaves Snake Way, he passes by Karin's tower on kinto'un en route to the battle vs. the Saiyans. Karin throws him two beans, one of which he eats to recover from the harsh trip down Snake Way.
2. When he arrives at the battlefield, Goku gives Gohan and Krillen each half a bean to help them heal from the fight against Nappa.

Usage #5 – 7 beans (Freeza saga)
1. Yajirobe makes another Senzu delivery to Goku's hospital, where he is sloooowly healing from his fight against Vegeta. Goku eats one, grabs the senzu bag, hops on kinto'un, and runs off to the spaceship for traveling to Namek.
2.&3. Goku uses two senzu beans during his 100X gravity training.
4. Goku stupidly hits himself with a kamehameha on the spaceship, and eats another one. 
5. When Goku arrives on Namek, he gives Gohan a bean to recover from the fight with Recoome.
6. When Goku arrives on Namek, he gives Krillen a bean to recover from the fight with Recoome.
7. When Goku arrives on Namek, he gives Vegeta a bean to recover from the fight with Recoome. He does this because he realizes that Vegeta is on their side in the fight, and he now trusts him to help them defeat Freeza. 

Usage #6 – 10 beans (Androids and Imperfect Cell)
1. Yajirobe makes yet another senzu delivery to the Z-fighters that were 9 miles Southwest of South City waiting for the androids. The first is used after Android #20 punches a hole through Yamcha's stomach.
2. During the Goku vs. Android #19 fight, everyone notices that Goku is weakening, and Krillen tosses him a bean. It has no effect, though, because the real problem was the heart virus.
3. After Vegeta beats Android #19, he realizes that a lot of his power was drained. Therefore, he asks Krillen to throw him a senzu. 
4. When Piccolo, Gohan, Tenshinhan, Krillen, and Vegeta are searching the desert for Android #20, #20 pulls a surprise attack on Piccolo and manages to take quite a bit of his energy. Piccolo recovers that energy with a senzu from Krillen.
NOTE: At this point it is revealed by Krillen that there are six senzu left.
5-8. After the fight with Androids #17 and #18, Krillen feeds Vegeta, Trunks, Piccolo, and Tenshinhan senzu beans because each of them was nearly unconscious. 
9-10. Piccolo and Tenshinhan each were beaten up by Cell later (Piccolo was nearly killed, blasted into the ocean by Cell. Tenshinhan was so weak after holding back Cell with all the Kikohos that he collapsed, and then Cell kicked him around), and each were given a senzu by Goku. 

Usage #7 – 2 beans (Cell's Perfection)
1. When Trunks leaves Kami's lookout to follow Vegeta to Cell, Goku gives him two senzu beans that Karin had kept from before. Trunks uses the first one a while later to heal Krillen. Krillen had recklessly attacked Cell out of frustration that Android #18, whom he loved, was absorbed.
2. The second senzu is used to heal Vegeta after his fight with Perfect Cell. Krillen had to move Vegeta away from the ongoing Trunks vs. Cell fight to feed him it, so Vegeta wouldn't jump right back into the action and become injured again.

Usage #8 – 8 beans (Cell Games)
1. Karin rushes, during the next ten days, to grow a few more senzu for Goku, and gives him the beans on his way to the Cell Games. However the first bean used is Given from Goku to Cell, after their fight. The purpose of this was to make sure the fight between Gohan and Cell would be fair. In case you've been keeping track, yes, this is the third act of utter stupidity by Goku in this one guide.
2-8. After Goku, Piccolo, Vegeta, Trunks, Tenshinhan, Yamcha, and Krillen were badly injured against the Cell Jrs, Gohan got the bag from Cell (who had stolen them from Goku earlier when he tried to eat one and interfere in his fight vs. Gohan) and fed Krillen a bean. He then gave the bag to Krillen and told him to heal everyone else.

Usage #9 – 3 beans
1. In the middle of the 25th Budokai, Goku uses instant transmission to go to Karin Tower to retrieve a senzu bean for Videl, who was badly beaten by Spopovitch in their fight. Karin gave him three. Goku gave one to Gohan, who in turn fed it to Videl.
2. Later, when Goku and Majin Vegeta are about to fight, and Gohan and East Kaioshin are about to go fight Dabura and Babidi, Goku tosses Gohan a senzu so that he will be able to fight better (he was still a little wounded from the first time that he fought Dabura).
3. After the Goku vs. Majin Vegeta fight, both warriors are wounded badly. Goku starts to pull out his bag, deciding to split the senzu bean so that they can both fight Fat Buu together. However, before he can do so, Vegeta knocks him out from behind, steals and eats the senzu, and goes through the portal in Babidi's ship to face Fat Buu.