Competition on the U.S. pharmaceutical market has never been more fierce. Not only are there innovations that spur new products all the time, but generic drugs are taking decisive steps onto the market, gaining a bigger share wherever they are allowed.

Because of the dynamics of the pharmaceutical market, companies know that their market shares can change significantly. A good example of that is GlaxoSmithKline and its best-selling respiratory drug Advair, which has lost some ground to competing products from AstraZeneca and Merck. Sales of Advair may also suffer because of the launch of Glaxo's new smokers' cough and asthma treatment, Breo.

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According to Bloomberg, Advair's prescription market share in the United States fell to 62.6 percent in the week leading to January 24, which is more than one percentage point lower than the share it held three months before. The drop could be attributed to growing demand for similar products, namely AstraZeneca's Symbicort and Merck's Dulera, which had a market share of 30 percent and 8.4 percent in January, respectively. Advair's total sales in 2012 came in at just over $4 billion in the United States alone.

According to London-based Bloomberg Industries analyst Sam Fazeli, Symbicort and Dulera have been gaining on Advair mostly because of their more aggressive approach to pricing and their rebating strategy. It is still early to evaluate the impact Breo might have, as it was only launched at the end of October.