BEIJING - China's top economic planner has fined two companies for monopolizing the sales of the base ingredient used to make drugs that treat high blood pressure.

The National Development and Reform Commission (NDRC) fined Weifang Shuntong and Weifang Huaxin -- both headquartered in the eastern province of Shandong -- together more than 7 million yuan (around $1.1 million).

A statement on the NDRC website said Monday that on June 9 the two companies signed a contract with the country's only two manufacturers of promethazine hydrochloride, the base ingredient for making high blood pressure-treating medicine.

The contract forbade the two manufacturers from selling the ingredient to a third party without approval from them.

The two firms then raised the base ingredient's price to as much as 1,350 yuan per kg from 200 yuan. Many producers of high-blood pressure-treating medicine were forced to halt production due to the high prices of the ingredient, according to the NDRC.

Their monopoly also caused a tight supplies of the medicine on the market.

The NDRC has ordered the two firms to stop their illegal activity and to dissolve the contract with the two manufacturers of promethazine hydrochloride.