Pepper spray could cause a person permanent damage such as permanent blindness if the pepper spray is not washed out of the persons eyes in time.
However if you get the pepper spray washed out of your eye in a timely manner then the most you'll suffer is temporary blindness and pain that will go away within a few days.
You don't want to get sprayed with pepper spray because it's a bad painful experience that will stop you in your tracks because you'll be in so much pain and cannot see that you will forget about what you're doing.
Pepper spray is one of the best ways to stop an attack that is not lethal and also works on dogs.
If the attacker wants to cause you permanent damage then it should be no issue if you cause them permanent damage.
If someone is attacking me they are lucky to be alive and that includes dogs that will attack me.
If a dog tries to attack me or someone else for no apparent reason then the dog is getting shot and possibly the human attacker if I have no other choice.