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it can be dangerous and usually not used for the good of the gods or universe, but for selfish reasons. I DO NOT SUGGEST trying it.

I've used someone elses blood twice., but they knew about it and they were ok with it.

the one time for one of my friends and he was going to iraq because of the marines and he wanted me to do the stringest protection spell that I could create, So I used some of his blood in it, but he was there and knew it, so I believe it was ok.

The second time, my dad and I did a binding spell which invovled mixing our blood pus a few different herbs and such in my cauldron. We had been seperated for 13 years and simply wanted to bind outselves together.

It's only when you use someones blood against their will or without their knowlegde thats bad, or animal blood is bad as well. At least, in my belief system.

I agree that it's dangerous and should be used sparingly. I'm interested in the scenarios in which it was used and why. I try to educate myself in as many forms of spell-craft as possible for a variety.

different tradition also have different views on the matter. For example, Voudun and Santeria (sorry if I've misspelled. It's late) both use animal sacrifce but that may be more of a ritual than magical significance.

I've never used blood, but I have used menstrual blood.

Actually it decomposing tissue. :/

Menstrual blood IS both tissue and the blood that provided the endometrium lining with nutrition during its building up process. It contains an enzyme that keeps it from clotting. Think of it as a disintegrating organ.
Regardless of its structure, menstrual blood has a lot of magical connotation for most of us. It comes from woman, the sacred womb and is tied (symbolically) to the moon. "O cup, o sacred chalice of the Goddess"
The acquisition of such blood is done without violence and the use is typically a way of binding self to spell (the "price" of a spell was previously mentioned in this thread). Many will use this blood in a coming of age ritual. It's also a really good fertilizer! Try some in a ritual for plant fertility and give some mixed with water to the plants in your home.
Standard blood is strong in that it is your life, really. It carries all that keeps you functioning through your veins and when you give some of it to a spell, it is a sacrifice of high honour. Voodoo/Voudou practitioners will mix blood with rum as a great sacrifice to their gods when asking for the big stuff.
Since the coming of AIDS/HIV and the realization that we have blood-borne diseases, the use of blood has been relegated to the truly hidden things witches do. We can use our own and that of those we trust.
Never ever STEAL someones blood.
If you practice a faith that uses animal sacrifice I URGE you to learn from leaders in your community so that it is done well and with respect to the animal (and local laws!).
I do not practice animal sacrifice, myself. If a spell needs blood, it will be my blood.
Speaking of which, donate blood, guys. It's a bit of your magic to save someone else. How much more could you honour your fellow humans than by giving of your life for theirs?