French police are searching German drug maker Merck KGaA's plant in Lyon as part of an investigation into complaints by patients about changes to its thyroid drug Levothyrox, a company spokeswoman says.
A Merck KGaA spokeswoman said on Tuesday the company was "ready to help investigators with their work". The search was carried out by a unit of the national police responsible for public health, which was not immediately available to comment.
Around 3 million people in France, 80 per cent of them women, use Levothyrox, a drug that works as a stand-in for the hormone thyroxine in patients suffering from hypothyroidism, a condition that affects the body's metabolism.
In March, Merck GKaA removed lactose from the drug to make it easier to tolerate and replaced it with citric acid and mannitol, a type of sugar alcohol, a move which thousands of patients said had resulted in severe side effects such as memory loss, hair loss, weight gain and palpitations.
The new formulation had been requested by the French medicines regulator ANSM in 2012.
In September, a prosecutor in Marseille launched an investigation into whether Merck KGaA had deceived patients with the change in the drug's formula.
Since then, Merck KGaA has restored the original drug to the market in the face of widespread media coverage in France. It will remain on shelves alongside the new formulation until other treatments are available.
