French financial prosecutors are investigating Sanofi for alleged market manipulation.

The preliminary probe opened in March involves alleged dissemination of misleading information related to the drugmaker’s financial communication, according to a judicial source who declined to be identified.

Sanofi said it isn’t aware of the investigation and its financial information is “accurate, precise and sincere.” The probe was first reported by French online media La Lettre, which says it relates to the launch of blockbuster medicine Dupixent in 2017.

The asthma and skin drug has since become Sanofi’s biggest seller. Sales of the antibody treatment could surpass $10 billion this year and go well beyond if the drug’s promise in treating a chronic lung disorder is confirmed.

The stock fell as much as 2.5% in Paris trading. Sanofi shares tumbled on Oct. 27 when the company issued a surprise forecast for lower profit next year and said it would spin off the consumer-health division.

(Updates with shares in fourth paragraph)