How does the Mangekyo Sharingan blind its wielder, and why are some wielders not blinded?


[A disadvantage of gaining the Mangekyo Sharingan is that it leads to
blindness over time.](https://i.stack.imgur.com/Ychk8.png)

[![enter image description
here](https://i.stack.imgur.com/Ychk8.png)](https://i.stack.imgur.com/Ychk8.png)
Source: Chapter 385, [Mangashare/Binktopia
scanlation](http://www.mangashare.com/)  
We have seen Madara, Itachi and Sasuke, all wielders of the Mangekyo
Sharingan, use Susanoo. All three have gone blind or have worsened eyesight at
some point*. Itachi and Sasuke have also been shown using Amaterasu which
makes the eyes bleed afterwards.

Danzo Shimura, Obito Uchiha and Kakashi Hatake, also wielders of the Mangekyo
Sharingan, on the other hand, have never used any of the above techniques and
have never shown any sign of blindness*. **Why didn't they go blind or have
worsened eyesight?**

**What causes the blindness effect of the Mangekyo Sharingan? How does the
Mangekyo Sharingan eventually blind its wielder?**

* * *

*Please correct me (or suggest an edit) if I'm wrong.  
Also, please feel free to suggest edits adding what chapters the events
mentioned above happened, so question answerers can review it.




An original Mangekyo Sharingan puts a strain on the eye. The ocular muscles
are placed under a constant strain due to the unique chakra the Mangekyo
consumes. That strain causes the user to gradually go blind.

Another person's eye doesn't (for some reason) produce the same strain. So the
user's vision is intact.

Let's review the list:

  * **Itachi** \- Awakened his Mangekyo on his own, going blind.
  * **Sasuke** \- Awakened his Mangekyo on his own, gone almost completely blind, then took Itachi's eyes, and got his vision back.
  * **Madara** \- Same as Sasuke.
  * **Kakashi** \- Remember that the eye didn't originally belong to him. His Mangekyo is more similar to an Eternal than a normal one.
  * **Obito** \- Presumably, it's because Obito has a high concentration of Hashirama Senju's cells. These constantly heal the eye, preventing it from going blind.
  * **Danzo** \- Same as Kakashi, the eye didn't belong to him. Although using Sishui's eye did put a huge stress on his chakra and body.




An original Mangekyo Sharingan puts a strain on the eye. The ocular muscles
are placed under a constant strain due to the unique chakra the Mangekyo
consumes. That strain causes the user to gradually go blind.

Another person's eye doesn't (for some reason) produce the same strain. So the
user's vision is intact.

Let's review the list:

  * **Itachi** \- Awakened his Mangekyo on his own, going blind.
  * **Sasuke** \- Awakened his Mangekyo on his own, gone almost completely blind, then took Itachi's eyes, and got his vision back.
  * **Madara** \- Same as Sasuke.
  * **Kakashi** \- Remember that the eye didn't originally belong to him. His Mangekyo is more similar to an Eternal than a normal one.
  * **Obito** \- Presumably, it's because Obito has a high concentration of Hashirama Senju's cells. These constantly heal the eye, preventing it from going blind.
  * **Danzo** \- Same as Kakashi, the eye didn't belong to him. Although using Sishui's eye did put a huge stress on his chakra and body.

