God say: ( Say : "I find not in the message received by me by
inspiration any (meat) forbidden to be eaten by one who wishes to eat
it, unless it be dead meat, or blood poured forth, or the flesh of
swine,- for it is an abomination - or, what is impious, (meat) on
which a name has been invoked, other than Allah's". But (even so), if
a person is forced by necessity, without wilful disobedience, nor
transgressing due limits,- thy Lord is Oft-forgiving, Most Merciful.)
[6:145]

Al Qortubi said: "Scholars agreed on blood being Haram (Islamically
prohibited), impure, non-edible and not beneficial."

Harms of taking blood on the health:
Blood carries different poisons, harmful wastes and compounds, as one
of its important roles is the transport of food metabolism resulting
wastes from the cells to be excreted; the most important ones being :
Urea, Ureic acid, Keratin and carbon dioxide. Blood also carries some
poisons from the intestines to the liver to be modified.

Once a big amount of blood is consumed, those carried harmful
compounds and those resulting from the digestion of blood itself get
absorbed and their levels increase in the body. This leads to an
increase in Urea's levels, which may result in brain dysfunction
leading to coma.

This is similaire to the diseased state in which bleeding of the
upper
digestion tract occurs and, normally, the accumulated blood is
removed
to prevent its harm to the body and the brain (eventually
coma).

Thus, blood contains harmful compounds even if taken from a healthy
animal in addition to its content of parasites and germs if taken
from
an originally ill animal.

Blood is a suitable environment for microbial growth:
It is medically known that blood is one of the best environments for
the growth of different types of microbes and as it contains the best
nutrition, it is used in laboratories to prepare microbial cultures.

Is blood suitable as a human food?
Blood content of digestible proteins such as albumins, globulins and
fibrinogens is little (8g/100ml) and the same is true with fats,
whereas it contains a big amount of haemoglobin, which is a complex
protein and so very hard to be digested, and in most cases the
stomach
can not stand it.

Moreover, if blood is coagulated its digestion becomes harder as
fibrinogen in this case is transformed into fibrin, forming a net
that
contains red blood cells and fibrin is known to be one of the worst
and hardest proteins to be digested.

Consequently, health experts did not consider blood in any way
suitable for human consumption.

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Reference  : Islamic Medicine Wonders Vol3 By Dr Mohammad Nezar
Aldeker