The sales of Viagra, formerly the nation’s best-selling drug to treat erectile dysfunction, have dropped by more than half in the five months since local generic drugs entered the market, market research company IMS Health Inc. said on Thursday.

IMS said that the sales of Viagra dropped continuously after the drug’s patent expired in May.

Between January and April, before its patent expired, Viagra had marked average monthly sales of 2.1 billion won ($2 million).

In September ― only four months after the patent expiration ― the monthly sales of Viagra had more than halved to 910 million won, as local generics such as Palpal Chewable tab of Hanmi Pharmaceutical Inc. increased market share.

Having a low price compared to the original, Palpal Chewable Tab, one of the best-selling generic versions of Viagra, rose in sales to 670 million won in September.

The original maker, Pfizer, sued Hanmi Pharmaceutical in October, claiming that it violated the design patent for Viagra. Pfizer said that Palpal Chewable tab consciously violates Viagra’s unique design ―blue and diamond-shaped.

Daewoong Co., CJ Cheil Jedang, and other big name pharmaceutical firms followed suit, reaping monthly sales of 120 million won and 100 million won respectively, with their genetic versions of Viagra.

The other 20-something generics of the drug marked monthly sales of less than 100 million won.

Viagra was first introduced to Korea in 2000 by the world’s largest U.S. pharmaceutical firm Pfizer Inc.

By Chung Joo-won (joowonc@heraldcorp.com)