Ironwood Pharmaceuticals Inc. of Cambridge and its European partner Almirall S.A. said Wednesday that Ironwood’s drug for irritable bowel syndrome with constipation has been approved by European regulators.

That same drug is about to go onto the US market, following approval by the Food and Drug Administration earlier this year. According to a recent Globe story, irritable bowel syndrome, or IBS, afflicts 30 million to 40 million Americans. Given the number of people affected, the drug has the potential to be a blockbuster, ringing up annual sales of $1 billion or more.