More than 9,000 people in France have reported serious side effects from a new version of the drug Levothyrox, which Merck is planning to put on the market in other European countries.
France in March became the first country where the new formula went on sale, but the government moved last month to restore the old version of Levothyrox, used by some three million people in France.
The reported side effects of the new version included cramps, dizziness and hair loss.
Merck confirmed that a search was carried out at its French headquarters in the eastern city of Lyon, saying it had been "expected" and that the company was cooperating with investigators. No arrests were made.
Health authorities are investigating 62 legal complaints over the drug, which were filed with the high court charged with public health matters in France's second city of Marseille late last month.
Limited quantities of the old version of the drug went back on sale in France yesterday.