(Adds further earnings details)

SAO PAULO, March 5 (Reuters) - Brazilian drugstore chain RD posted on Tuesday a 5.9% decrease on its adjusted net profit for the fourth quarter from a year earlier, amid higher selling expenses and sales growth slowdown.

The firm posted 283.3 million reais ($57.14 million) adjusted net profit for the quarter ended in December.

Its net revenue was at 9.6 billion reais in the quarter, up 14% year-on-year, with gross revenue increasing in all products segments. Same stores sales rose 7.7% - a year earlier it had risen 16%.

Meanwhile, the firm's sales expenses rose 19.8% in the quarter.

It said lower level of sales growth "resulted in a momentary loss of operating leverage", adding "with the normalization of our sales growth, as observed in January and February, we expect a return to the level of expenses posted recently."

RD, which is Brazil's largest drugstore chain by number of stores, said preliminary numbers showed same stores sales rising 11.6% in January and February year-on-year.

The company's adjusted earnings before interest, taxes, depreciation and amortization (EBITDA) was at 614.5 million reais in the quarter, increasing 2.5%.

Its margin measured by the gross profit/gross revenue ratio was at 28.1% from 28%.

($1 = 4.9581 reais) (Reporting by Andre Romani in Sao Paulo; additional reporting by Patricia Vilas Boas; Editing by Steven Grattan)