Calcutta: Paracetamol, the most commonly prescribed medicine for dengue and "fever of unknown origin", has vanished from the inventories of many pharmacies in town.

Paracetamol 650mg, which a doctor said was the go-to medicine for treatment of high-grade fever, has sold several times more during this dengue outbreak than at any time over the past few years.

Pharmacists from Kasba in the south to BK Pal Avenue in the north said demand for paracetamol was currently so high that distributors were unable to keep up with it. Calpol 650mg has been hard to find in medicine stores over the past two weeks.

Other paracetamol variants like Crocin and Paracef are also in short supply.

"The 650mg tablet is the most commonly prescribed paracetamol because the dose for high fever is 10-15mg per kilo of body weight and a normal adult weighs around 65kg," said Amitabha Nandy, director of the Centre for Studies on Infection and Immunity.

Three to four such tablets a day are prescribed to a patient diagnosed with dengue within two to five days of coming down with fever. The same dosage applies to a patient with fever of unknown origin, a term used to describe illness with symptoms of dengue but no confirmation through a blood test.

Going by the standard arithmetic, a patient with high fever would need a strip of 10 pills for three days.

"Until August, we were selling 25 strips a day. The numbers started rising in September. We have been selling 60 strips of all variants of paracetamol a day at least for a month," said Sankar Adhikary, an employee of the Well Med pharmacy on Sambhunath Pandit Street.

The state government last updated dengue statistics in mid-October, by which time 20,000 cases had been confirmed. The death toll stood at 40.

With the outbreak showing little sign of abating, the paracetamol queue has only grown. A south Calcutta resident whose wife and daughter were diagnosed with dengue bought five strips of Calpol 650 in three weeks. "You never know when you might need a paracetamol. If someone at home comes down with fever, we will have at least some tablets to use," he said.

Arijit Saha, a pharmacist at LB Pharma opposite RG Kar Hospital, said his store would sell around 13 strips of paracetamol a day around this time in 2016. This year, the store has been selling 40 strips a day.