Re: Why does morphine make your blood feel warm?

Morphine belongs to a class of potent narcotics which cause an immediate release of histamine, a chemical "messenger," when injected. Histamine is a potent vasodilator (it increases the diameter of blood vessels which in turn causes an increase in blood flow and is often perceived as an increase in warmth). In most patients this effect is beneficial, but in a small percentage of the population injection of morphine may lead to a severe allergic reaction against the drug. In a medical setting morphine is a valuable, reliable, inexpensive and predictable drug. Its main drawback is the distinct possibility of addiction with repeated use.

Reference: Goodman and Gillman, Pharmacological Basis of Therapeutics, 9th edition.