One of the more curious side effects of heroin use is an itchy feeling that spreads across small patches of skin. Heroin related itching typically occurs around the nose and upper part of the face. However, higher doses may affect more generalized areas, such as arms, legs and genitalia.

What is itchiness?

Itchiness is a subjective, irritating sensation arising from the superficial layers of skin that provokes an urge to scratch. Scratching is a reflex response to an itch. The command to start scratching comes directly from the spine. We know this because researchers discovered that animals with spinal cords severed from the brain still had a scratch reflex.

Heroin itching

Itchy skin is one of the most prevalent adverse effects of heroin use.  Many scientists believe that heroin induced itchiness is an unavoidable side effect common to all opiates, and for the most part this is true.
But what causes the heroin itch?

Heroin itching and scratching

It’s simple really. After heroin gets into the bloodstream, it enters the brain and spinal cord. There it attaches to proteins atop nerve cells called opioid receptors. The most common type of opioid receptor is the mu-opioid receptor (MOR). Now each MOR has several variants, which are called isoforms. Isoforms are like your hands and feet. Maybe you use your left foot to kick things and your right hand to open things. In this way, each hand or foot or isoform does something different. There is one MOR isoform in particular, called the MOR1D, that is the hand that opens the door to itchiness. 
When heroin bumps into MOR1D receptors, it activates those receptors, which subsequently sends messages to other receptors atop other nerves. When MOR1D receptors are activated in proximity to GRPR receptors, some of those messages activate GRPR receptors.
Research has shown that GRPR receptors are itch specific receptors located within the spinal cord.  GRPR receptors are found in a very small population of spinal cord nerve cells. Scientific research, led by Zhou-Feng Chen, Ph.D., found that laboratory mice that lacked GRPR, scratched much less than normal mice, when given itchy stimuli.
The bottom line is that heroin administration sets off a cascade of events. The first of which, is that heroin binds to and activates MOR1D receptors, which subsequently sends out new messages, called neurotransmitters, that activate GRPR receptors, and this results in an itch sensation, that ultimately leads to scratching.

how heroin works