Does drawing blood from the human body by injection break the fast or not?

You must make up for the day if the amount of blood drawn is customarily large. You do not have to make it up if it is small. This is to avoid dispute and out of caution. Drawing blood for analysis does not invalidate the fast because it is something necessitated by need and is not among the legal nullifiers of the fast. As for donating a large amount of blood, it invalidates the fast by analogy with cupping (hijamah), whereas a small amount does not break the fast.