Reckitt Benckiser, the Anglo-Dutch household products giant, is to enter the over-the-counter medicines market in the United States after agreeing to buy Adams Respiratory Therapeutics for $2.3 billion (£1.1 billion) in cash.

The acquisition of the Nasdaq-listed maker of Mucinex, the market-leading cough remedy, is Reckitt’s first big foray into the American consumer health sector.

Mucinex will become one of Reckitt’s “Powerbrands” — the portfolio of key, fast-growing products that the Slough-based group promotes heavily around the world. Adams will also give Reckitt an American distribution network through which it can market its existing over-the-counter drugs.

Reckitt, whose brands include the cleaning products Cillit Bang, Vanish and Lysol, sharply ramped up its presence in over-the-counter medicines last year when it acquired Boots Healthcare International (BHI), the maker of Nurofen and Clearasil, for £1.9 billion.

Bart Becht, the Reckitt chief executive, said that he expected to double the size of the Adams business within five years and to increase margins to about 35 per cent, in line with those of BHI.

Adams, which is active only in the US, had sales of $332 million in the year to June 2007, up from $14 million in the year to June 2003. Most of that growth came on the back of Mucinex, also known as guaifenesin, which was approved by the US Food and Drug Administration in 2002 as a treatment for chest congestion. Adams’s only other brand is Delsym, another cough remedy.

Adams’s latest quarterly results showed a 22 per cent increase in net sales, to $110 million, while net income came in at $29.2 million, up from $16.2 million a year earlier.

Reckitt is planning to offer Mucinex outside the United States from 2010, Mr Becht said. He added that Adams had a pipeline of new products in related fields.

Shares in Reckitt closed up 2 per cent in London, at £29.83.

However, Adams and the access that it gives to the US market is coming to Reckitt at a price, analysts said. Reckitt is paying more than six times sales for the group, compared with the 3.6 times that it paid for BHI.

Reckitt has agreed to pay $60 a share in cash for Adams, a 37 per cent premium to Friday’s closing price. Adams’s shares rose more than 35 per cent in pre-market trading in New York on news of the deal. For the past seven years Reckitt has achieved a 7 per cent increase in net sales every year, against an industry average of 4 per cent. Net profits have nearly quadrupled since 1999, from £200 million to £768 million in 2006. In October, the FTSE 100-listed group said that the integration of BHI was ahead of schedule. It expects cost savings from the deal for the year to be £5 million higher than previously expected, at £35 million.

Reckitt said that there would be a one-off charge of $60 million for integrating and restructuring Adams.

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