Parents have ben warned that some head lice lotions used on thousands of children could catch light and cause serious burns.
Adults are being urged not to smoke around treated children or let them near naked flames.
Even being close to a cigarette the morning after having lotion applied could be dangerous, experts fear.
The Medicines and Healthcare products Regulatory Authority – which polices the safety of medical products – has issued an alert to pharmacists.
It wants them to highlight the risks to parents buying the over-the-counter lotions.
At least ten Brits have suffered serious burns in the last decade after head lice lotion set their hair alight.
Eight children and adults were seriously burned after using top-selling brand Hedrin, which costs £12 per 150ml bottle. Another two caught fire using rival brands Full Marks and Nyda.
Most cases involved youngsters injured when an adult lit a cigarette near them.
One woman with head lice was left scarred for life after simply bending down to adjust a gas fire.
The lotions all contain flammable liquids to help kill off lice – which affect more than one in ten primary school children in the UK.
Hedrin packaging warns: “Keep hair away from source of ignition, especially naked flames and burning cigarettes.
“Treated hair can readily burn if ignited.”
However, the MHRA fears parents are still unaware of the dangers.
It first warned of the risks in 2007, when lotion manufacturers were ordered to carry warnings on packs but accidents are still happening.
Only last year tragedy struck when a child being treated with flammable lotion came into contact with a naked flame – “resulting in very serious burns”, the MHRA said in a notice to pharmacists.
The UK market for head lice products is worth nearly £20m a year.
The tiny insects, about the size of a sesame seed, feed on blood from the scalp. They attach their eggs – or “nits” – close to the skin and can cling so tightly to hair that even a thorough wash will not dislodge them.
A spokesman for Hedrin said: “Customer safety is of utmost importance.
“Hedrin four per cent lotion carries a prominent warning to keep hair away from sources of ignition, especially naked flames and burning cigarettes.
“The rest of the Hedrin range is not associated with these issues but we do always advise keeping hair away from naked flames.
“We recommend that users check the risk and benefits when using pharmacy products and always speak to the pharmacist for additional advice.”
The Proprietary Association of Great Britain, which represents over-the-counter medicine manufacturers, said it welcomed the MHRA guidance to pharmacists.
Chief executive John Smith said: “It is important to reassure people that if used in accordance with the clear on-pack instructions and the patient leaflet information inside, head lice treatments are an appropriately safe way to eliminate head lice and eggs.