The agency confirmed the assessment this week after carrying out its own analysis on the new drug.
Levothyrox - manufactured by laboratory Merck - is the most widely-used treatment option in France for patients suffering from underactive thyroid function and other thyroid conditions, which can cause weight gain, depression, circulation problems, coldness, and extreme fatigue.
The new formula of the drug, which was issued in summer last year, has been highly contested by patient association l’Association Française des Malades de la Thyroïde (AFMT).
The group has accused the formula of “anomalies” in its composition, of being ineffective, and causing side effects including headaches and dizziness, and said that at one point, the association was receiving 1,000 complaints a day from patients suffering on the new formula.
The AFMT claims it has completed its own independent analysis of the new formula, but this has been called “scientifically unfounded” by Levothyrox producer, Merck.
In September last year, health minister Agnès Buzyn was forced to bring back the old formula into pharmacies pending further investigation into the furore - with almost half of the 130,000 old boxes selling out in just two days.
Ms Buzyn has also faced calls to resign over the issue.
But now, the ANSM has backed Merck’s view, saying in a press release its new analyses “confirm the good quality of the new formula of Levothyrox”.
The statement continues: “[The analyses] show quantities of [active ingredient] levothyroxine at comparable levels between the old formula and the new, with the latter therefore not ‘under-dosing’”.
This directly contradicts the allegations made by the AFMT, which said that its own analyses revealed “a level of thyroxine seriously inferior to the necessary requirements”.
It also said that there was also an “abnormal level of dextrothyroxine” in the drug - a controversial molecule sometimes used in cholesterol-lowering medication.
Yet, the ANSM has said that there are similar levels of dextrothyroxine - just 0.1%, it said - in both the old and new formulae.
The ANSM statement concluded: “The presence of dextrothyroxine in these medicines is to be expected for this type of substance, and does not in itself represent a defect in quality or risk to health.”
The agency acknowledged the existence of the AFMT analysis, but said that it could not be taken seriously.
It said: “The absence of reference samples or the name of the laboratory used, and a demonstrably-inappropriate methodology of the finished product, makes it impossible to use the analysis by the AFMT.”
In June, the AFMT issued a statement on the new formula, saying: “At this stage, our association does not claim, with just one single study, to have indisputable ‘proof’ - but significant proof.”
It added: “We have seen cancerous patients [using the new formula] finding themselves severely under-dosed with thyroid hormones; we have seen cases in which patient cancers have come back after years in remission.”
In France, over three million people rely on some form of thyroid medication.
Merck's Levothyrox previously had a near-monopoly in the country, but now there are four others available: L-thyroxin Henning, Thyrofix, L-thyroxine SERB, and TCAPS.
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