Why snakes bite and how venom works 

When a snake bites and injects venom, usually its aim is to immobilise its natural (food) prey.

As humans are far too large for a venomous snake to eat, most snake bites occur when the snake is provoked into acting in self-defence.

In most cases, the snake is provoked by accident for example, when a person accidentally steps on a snake while out walking. However, sometimes a snake bites after being deliberately threatened and frightened by someone:

  • kicking it
  • striking it
  • trying to pick it up

Snake bites that involve foreign (exotic) snakes kept as pets usually occur when someone handles or 'plays' with them, often after drinking too much alcohol or taking recreational drugs.

Snake venom

Snake venom contains many different toxins (poisons) evolved to kill or immobilise the snake’s prey. There are four main types of snake venom toxins:

  • haemotoxins  affect the circulatory system (heart and blood)
  • neurotoxins affect the nervous system at the places where nerves connect to muscles
  • cytotoxins damage and kill tissue cells (such as skin) causing blood and plasma (the clear fluid in blood) to leak into the tissue near the bite
  • myotoxins destroy muscle tissue both at the site of the bite and generally throughout the body

The four types of toxins are discussed in more detail below.


Haemotoxins destroy red, oxygen-carrying blood cells, damage the lining of blood vessels and disrupt the blood's ability to clot.

They can also cause a drop in blood pressure, which can result in tissue and organ damage, loss of consciousness and death.


Neurotoxins block or damage nerves where they connect to muscles, preventing the nerve signals getting through.

This causes paralysis and symptoms such as muscle weakness throughout the body and swallowing and breathing difficulties that can lead to lack of oxygen (respiratory failure) and death.


Cytotoxins cause swelling, bruising, blistering and gangrene (death of tissue cells) near the location of the snake bite. This may require plastic surgery or, in severe cases, amputation.


Myotoxins damage muscle cells, causing pain and muscle weakness.

They may also damage your kidneys, which filter waste products from your blood, causing the flow of urine to decrease or even stop. The urine may become dark brown or black.


Adders (top) have distinctive, black zigzag markings along the length of their bodies. Grass snakes (bottom) have a yellow and black 'collar' behind their head

Is it an adder?

The adder is common throughout mainland Britain and some islands off the west coast of Scotland.

Appearance of adders (top photo):

  • adders have a distinctive, dark zigzag stripe down their back
  • they are quite short, up to a maximum of 75cm (2ft 6in) long
  • they have a large head and slit-shaped pupils
  • males are usually grey with black markings
  • females are usually brown with darker brown markings
  • however, sometimes they can be silver, yellow, green or completely black

Adders can sometimes be confused with:

  • Grass snakes (bottom photo) which are longer (up to 120cm, or 3ft 11in) and are greenish, grey or brown with black flecks or bands, fast moving and often found near water.
  • Slowworms (legless lizards)  which are shorter (up to 50cm, or 1ft 8in), have tails that break off easily, are uniformly brown or grey and sometimes have a straight dark line down their backs.
  • Smooth snakes which are grey or brown with black dots down their back. They are only found in southern England and are rare.

Page last reviewed: 11/08/2014

Next review due: 11/11/2016