Walkers have been warned to leave snakes in the countryside alone after dozens of incidents of bites caused by people picking up venomous adders.
People sought advice over adder bites 196 times between 2009 and 2011, figures from the Health Protection Agency's National Poisons Information Service (NPIS) showed.
In around half the cases, a person had picked up an adder, the only venomous snake living wild in England, Scotland and Wales.
When adder bites deliver venom, it can cause localised pain, tenderness, swelling and bruising which can spread.
If a child is bitten, the effects can be seen across their whole body.
The HPA said almost all poisonings from adder bites produced relatively minor effects, but more serious cases could involve kidney failure in children, abdominal pain, nausea, vomiting, serious heart effects and even - very rarely - death.
Professor Simon Thomas, director of NPIS Newcastle, said: "Adder numbers have decreased in recent years so they are rare but still present in certain areas.
"They usually keep well out of sight, but in the summer months are active because the weather is warmer.
"Because they are well camouflaged, people can accidentally tread on them, which is when they can bite. They can also bite if picked up."
And he said: "The bite can have very nasty effects, especially in smaller children - so it's best to take care when out walking, wear appropriate footwear for the terrain and do not handle any snakes.
"Sometimes the venomous adder can be mistaken for non-venomous species such as the grass snake or smooth snake, making people think it is safe to pick them up."
The experts said bites could occur between February and October, most commonly in the warmer summer months.