Doctors have warned teenagers about the dangers of sniffing mothballs after two teenagers fell ill through the habit.
An 18-year-old French girl had to be hospitalised when she developed scaly skin on her legs and hands, unsteadiness and mental sluggishness.
Medics were initially puzzled, especially as her twin sister displayed similar, but milder, symptoms.
But the New England Journal of Medicine reports that days later, it was found the mothballs were to blame.
Any form of volatile substance abuse is incredibly dangerous
It was discovered that the girls had been using the mothballs as a recreational drug when doctors found a bag of mothballs stashed in her room while she was being treated at the Hospital of Timone in Marseille.
Both girls had been "bagging" - inhaling mothball fumes - after encouragement from classmates.
The twin who was sickest had also been chewing half a mothball a day for two months.
She continued her habit in hospital because she did not think her symptoms were linked to the mothballs.
The balls, used to prevent moths getting into clothes, contain paradichlorobenzene (PDB), a substance also found in air fresheners and insect repellents but which can cause liver and kidney failure, and severe anaemia.
The doctors who treated the girls said the habit was "dangerous" and most likely under-reported in medical literature.
The sickest teenager took six months to recover fully.
Her twin, who had only been "bagging" for a few weeks, recovered after three months.
Writing in the journal, Dr Lionel Feuillet said: "Substance abuse by youths is a major public health concern.
"PDB is derived from aromatic hydrocarbons, which form one of the families of volatile substances that are commonly abused."
He said only three cases of getting high using mothballs.
But he added: "Since young people usually deny practicing self-intoxication, the incidence of this type of recreational activity is probably underestimated."
He said clinicians should be aware of the symptoms.
A spokesman for the UK organisation Drugscope said: "We are not aware that sniffing or eating mothballs is an issue in the UK.
"However, any form of volatile substance abuse (VSA) is incredibly dangerous.
"About a third of the young people who die from VSA die the first time they try it."
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