Demand for Viagra falls during big sporting events, suggesting men prefer sport to sex, researchers said yesterday.

During the last World Cup the number of Viagra prescriptions fell by more than half from 27,000 a month — the biggest drop recorded.

Isis, the independent health research firm, said either the excitement was a natural stimulant, or people were so involved in football that they did not have the inclination for sex, preferring to concentrate on their team. Doctors suspect the latter.

The figures are expected to be studied closely by the drugs companies who will see that sales are seasonal in a way they had not accounted for.

Isis Research uses a panel of 400 doctors across the country to look at trends.

The massive drop in Viagra and similar prescriptions last June was the only such fall of that size in the past two years. It was so significant that doctors could not put it down to holidays, the Jubilee or other summer influences, said Isis.

Amy Ridler, a researcher at Isis, said: “After years of drug development, it seems that the fortunes of the newest products in the erectile dysfunction market may be affected not only by advertising and sales calls, but by a game of football.”