An older version of a thyroid drug made by Merck Serono is planned to be reintroduced in France after more than 9,000 of the country's three million users complained of serious side effects such as cramps, headaches, dizziness and hair loss from a new formula launched in March of this year.

The French drug approval agency Agence Francaise du Medicament had asked the German company to develop the new formula, which is said to be stabler than the old version.

Merck said the new version of Levothyrox is set to be approved in other European countries.

Levothyrox is prescribed to treat underactive thyroids or following surgery for cancer of the organ. News reports said France is the world's leading consumer of this treatment and the first country in which the new version was introduced.

Only one alternative is currently available in France, L-Thyroxine, manufactured by a Sanofi subsidiary. Health Minister Agnes Buzyn told France Inter radio that "alternatives, other brands, other drugs" will be available in a month.