Prescribed to more than three million people in France to replace the hormones that the thyroid no longer produces, Levothyrox has so far been an uneventful drug. But, following a change of formula in March 2017, it is at the heart of a vast health crisis. Dizziness, fatigue, memory loss, palpitations, muscle or joint pain, some patients experience a nightmare. The controversy grows. More than 17,000 reports of adverse effects have been recorded by the National Medicines Safety Agency. Why did she ask the Merck laboratory, now sued, to change the formulation? For several years, specialists have also been sounding the alarm bells against the excessive screening and overdiagnosis of certain thyroid cancers,