Drug approval in Japan often takes years.

It will only be available with a doctor's prescription and is not covered by health insurance, but sales are still expected to be massive.

"Japan is the world's second-largest market for pharmaceuticals, so of course we look forward to strong sales in line with this," said Takaichi Hirota for Pfizer Pharmaceuticals, the company that manufactures Viagra.

According to some reports, the number of men in Japan who suffer from impotence is as many as seven million.

"Erectile dysfunction is a serious condition and those who suffer from it tend to suffer alone and in silence. But now there is help for these people," Mr Hirota added.

'Viagra tours'

In Japan, a 25mg tablet will cost 1,100 yen ($9) and a 50mg tablet 1,300 yen ($11), roughly in line with prices in the United States.

Men must also pay for pre-prescription urine and blood tests, which will cost an estimated 10,000 yen ($85) or more.

Previously, the small blue pills could be bought illegally on the black market for 6,000 yen ($50) a tablet.

Japanese men have even gone on 'Viagra tours' to Hawaii to buy the drug and bring it home.

In contrast with the swift approval of Viagra, the Japanese authorities recently delayed a long-awaited decision on whether to legalise the sale of the contraceptive pill - 34 years after it was introduced in other countries.

Japan is the only country in the United Nations where the pill is not available as a form of birth control, although it is used for medical reasons.