In late March 2017, the Merck laboratory launches a new formula of Levothyrox, a drug used by three million French to compensate for the malfunction of their thyroid gland or after thyroid surgery.

Two months later, many patients complain of significant side effects. A petition is launched to request the return to the old formula of the drug. And a toll-free number was set up at the end of August by the National Agency for Drug Safety (ANSM) * to answer patients' questions. The number of calls quickly exploded: more than 150,000 in a few days. However only certain excipients have been changed in this medicine.

What is an excipient?

Theoretically, an excipient is a component that has no pharmacological effect . It serves as a support for the active ingredient and makes it possible to give a taste, a form and a stability to the drug. But not all excipients are inert.

Some are qualified as "well-known" excipients because they can be poorly tolerated . "Aspartame, for example, is a source of phenylalanine, a substance contraindicated in people with phenylketonuria," says ANSM. Peanut oil, Peruvian balm, wheat starch or lactose are also likely to cause inappropriate reactions in sensitive people.

In theory, modifying the excipients has no impact on the efficacy and tolerance of the treatment. However, they sometimes change the absorption of the active ingredient, up or down .

Therapeutic margins sometimes narrow

"The effectiveness of a drug is based on its active ingredient, not on its formulation," says Professor Jean-François Bergmann, head of the department of internal medicine at Lariboisière Hospital. This is why generics have the same effects as the standard medicines of reference. There is bioequivalence. They are identical twins.

But there are some drugs with narrow therapeutic margins where the slightest variability of dosage can induce a slight decrease in efficacy or - conversely - a tiny overdose . Levothyrox is one of these rare pharmaceutical specialties **, since the thyroid balance is sensitive to very small dose variations. This is why the majority of endocrinologists place the prescription on the prescription "Non-substitutable" when they prescribe Levothyrox so that the pharmacist does not deliver a generic instead.

Side effects explainable?

Symptoms complained of by patients following the reformulation of Levothyrox strongly suggest hypothyroidism (hormone deficiency in the blood): severe fatigue, weight gain, vertigo, hair loss, constipation, depressive disorders ... In other words, the excipients (replacement of lactose with mannitol and addition of anhydrous citric acid) do indeed seem to modify the absorption of the active substance in some patients , even if the latter is unchanged.

It's as if the new Levothyrox was a generic of the old Levothyrox. Dosage adjustments may therefore be necessary.

In case of lasting problems with the new formulation, consult your doctor to prescribe a thyroid assessment and proceed if necessary to a change of prescription.

* The Levothyrox Medicines Agency's toll-free number: 0 800 97 16 5

** Antiepileptics are also drugs with narrow therapeutic margins. According to Prof. Charles-Joël Menkès, author of a report of the Academy of Medicine on generics, "some generic antibiotics also seem to have less efficiency and a longer period of action." But for the vast majority of pharmaceutical specialties, generics have the same action as the reference drugs.