The drugs: Advair/Seretide
The company: GlaxoSmithKline
Estimated worldwide sales 2012: $8 billion

Despite losing its patent coverage in several countries, GlaxoSmithKline's ($GSK) Advair franchise is still putting up high sales numbers. That's because the drug, which combines two lung remedies in one inhaler, has proven difficult to replicate.

Consider Greece's Elpen, which won approval from Swedish regulators for its knock-off version in June 2011. That approval opened the way for Elpen's version to launch in Germany, Italy, Portugal, Hungary and the Czech and Slovak Republics, along with Greece. But the inhaler device was different enough to prevent easy substitution for the Glaxo drug, sold as Seretide in Europe, so the impact on sales has been limited.

Still, Advair revenues have slumped slightly. Used to treat asthma and chronic obstructive pulmonary disease, the drug raked in $8.7 billion in 2011, as opposed to the estimated $8 billion it's expected to bring in this year. Besides the generic competition, such as it is, the franchise may be suffering from stepped-back marketing efforts, analysts say. Glaxo recently settled longstanding marketing allegations with the Justice Department, including claims that it pushed Advair for use in patients with mild asthma, when it was only FDA-approved for more severe cases.

Advair should watch out for U.S. generics, too. In hopes of mimicking the Advair/Seretide Diskus inhaler, Mylan ($MYL) paid Pfizer $17.5 million plus milestones for drug delivery technology last year.

For more:
Glaxo's U.K. plant keeps inhaler production in-country
GSK sales grow, thanks to emerging markets, Japan
GSK chief was right about tough-to-copy Advair