|Adders are the only venomous snakes in the UK.|
The UK's only venomous snake is the adder and up to 100 people are bitten by it each year, most of them in summer.
Bruising and swelling are the most common symptoms of a bite and these can go from the hand to the chest wall or from the foot to the groin.
Antivenom can reduce the severity of the injury but it needs to be given quickly or the patient could be left with mobility problems requiring long-term physiotherapy.
Fourteen people sought hospital treatment for adder bites during July, but some of those left it too late.
Professor Michael Eddleston, of the University of Edinburgh, said: "We're seeing an increase in people who have been bitten coming to hospital after 24 hours or so, when their swelling is already quite severe.
"This puts them at risk of painful mobility problems.
"Treatment is much more effective if given early and we would urge victims to get to hospital as quickly as possible.
"Not all will require antivenom but those who do will get it quickly, limiting the extent of the swelling and the long-term complications."