Are Adders Dangerous? Real Risks of Britain's Venomous Snake

How dangerous is an adder bite?

The common European adder (Vipera berus) is Britain’s only venomous snake and the most widespread viper in the world. Its venom mainly damages tissue and blood vessels around the bite, causing pain and swelling. It is not one of the world’s strong venoms: laboratory tests show adder venom is considerably less toxic than that of many other vipers, and the snake injects only a small amount, estimated at roughly 6–18 mg in adult snakes.

That does not make a bite trivial. Swelling can spread along the whole limb, and some people develop vomiting, dizziness, a drop in blood pressure or a severe allergic reaction. Young children, older adults and people with heart or lung conditions are at greater risk, because the same amount of venom has a bigger effect on a small or weakened body. Recovery from a serious bite can take weeks, and some adults report pain and stiffness for months.

Not every bite delivers venom. Like many vipers, adders sometimes give a “dry” bite with little or no venom, but there is no way to know which kind it was without medical assessment. That is why doctors treat every adder bite as an emergency.

How many people die from adder bites?

Very few. Britain recorded 14 deaths from adder bites between 1876 and 2005, and the last was a five-year-old child in 1975. In the same period, millions of people walked across heaths and moors where adders live. A 2012 review of snakebites across Europe, including European Russia and Turkey, estimated around 8,000 venomous bites a year but an average of only about four deaths, mainly because hospitals and antivenom are widely available.

Sweden, where adders are common in gardens and summer cottage areas, sees around 1,300 bites a year, and a minority of those patients need to stay in hospital. Occasional deaths still happen in Europe, usually when treatment is delayed or the person was already in poor health.

For comparison, dog bites send thousands of people to hospital in England every year, far more than adders ever do.

When and why do adders bite?

Adders are not aggressive. They rely on camouflage and usually slip into cover when they feel footsteps through the ground. Most bites follow one of three situations:

• Picking up or handling an adder, often by someone who thinks it is harmless or wants a photo. Bites on the hand or arm usually happen this way.
• Treading on or sitting next to a basking snake, especially when walking barefoot or in sandals on heaths, dunes or long grass.
• Reaching into vegetation, log piles or stone walls where an adder is resting.

Most bites happen between March and October, when adders are active, with a peak on warm spring and summer days. In spring, adders bask in the open for long periods after leaving hibernation and can be slow to move until they warm up, which makes them easier to step on.

When an adder feels threatened, it often gives a warning first. It may hiss loudly, flatten its body and pull its neck into an S-shape. Backing away slowly at that point almost always ends the encounter.

How to stay safe around adders

The safest approach is simple respect. Adders want to avoid people, and a little care avoids nearly all bites:

• Watch adders from a distance of a few meters and never try to touch, pick up or move one.
• Wear boots and long trousers on heathland, moorland and dunes, and keep to paths.
• Look before you sit down on banks, rocks or logs, or put your hands into dense vegetation.
• Keep dogs on a leash in adder country in spring and summer; dogs are bitten more often than people.
• Teach children to recognize the zigzag pattern and to step back and call an adult.

If someone is bitten, call 999 (UK), 112 (EU) or 911 (US) and seek emergency medical help immediately, even if the bite seems minor. Do not try to catch or kill the snake; a photo taken from a safe distance is enough if one is possible.

Is the adder the most dangerous snake in Europe?

No. Several southern European vipers have stronger venom or inject more of it, and their bites are usually more serious. The nose-horned viper of the Balkans and the asp viper of France, Italy and Switzerland are the main examples. See our guide to venomous snakes in Europe for where each lives.

In Britain, though, the adder is the only venomous snake. The two other native snakes, the barred grass snake and the smooth snake, are harmless, and the slow worm is a legless lizard. If you are not sure what you have seen, our snakes in the UK guide and adder vs grass snake comparison explain how to tell them apart.

Myths about adder bites

• “Adders attack people.” They do not chase or ambush people; they bite in self-defense.
• “A bite is always deadly.” Deaths are extremely rare in modern Britain, and most people recover fully with medical care.
• “A small bite with no pain is safe.” Symptoms can be delayed for hours, so every bite needs checking at a hospital.
• “Adders should be killed to keep people safe.” Adders are protected in Britain under the Wildlife and Countryside Act 1981, and killing or injuring one is illegal. They also help control voles and mice.