ZURICH—Like many Western firms, Procter & Gamble Co. is eyeing new opportunities in Iran following the relaxing of trade sanctions earlier this year. But for a Swiss subsidiary of the U.S. consumer goods giant, Iran already is a very familiar market.

Starting with market research in 2003, and culminating with more than $100 million in sales in the year ended in mid-2010, Geneva-based Procter & Gamble International Operations SA’s business in “Parthia”—an in-house byword for the Middle Eastern country—was a success...