The pharmacy was at the centre of the investigation into the horses' deaths in mid-April after its own inquiries confirmed that it had provided a supplement in which the strength of an ingredient was incorrect.
The mix contained too much selenium, which resulted in the horses' deaths after they were given the supplement. The animals were part of the Venezuelan-owned Lechuza Caracas team.
US media report that Lab worker Sheila Harris alleges in a complaint filed on July 16 that she was fired for speaking with investigators, despite her termination letter stating it was a restructuring move aimed at improving effiencies.
Franck's Pharmacy denies the allegation, saying it encouraged staff to co-operate with those conducting the investigation.