Ways friendships can be unhealthy

This list is not comprehensive. These may have substantial overlap, and are not offered in any other order than how they come to mind:

1. I am  closer to my friend than to my spouse. This automatically makes the friendship unhealthy. This can be an issue regardless of the gender of the friends. This might also be an example of others.
2. The relationship doesn't have clear, appropriate boundaries. There are too many ways for this to be the case to list them all. But if one or both friends are continually either taking advantage of the other or wondering if they've overstepped, the friendship isn't healthy.
3. One of the friends doesn't respect the other's boundaries. You cannot be closer to someone than they want to be. And you cannot solve another person's issues. 
4. The relationship can be out of balance. It can be more important to one friend than to the other. This can also lead to boundary issues, but it's a separate concern of its own.
5. It can be enabling. There could be elements of the friendship that promote other unhealthy behaviors. Perhaps the friends drink together, and one of them becomes an alcoholic or drug addict. One friend might mistreat their spouse or family and unwittingly use the friendship as part of their cycle of mistreatment or abuse. One of the them might take validation from the friendship that keeps them from addressing an outside issue on which they need to work. (singular "they" applied.)
6. I'm sure there are more . . .