Generic drugs maker Acrux has been poking an $8.7 billion bear — and it’s hit back with a law suit in the US.

Acrux (ASX:ACR) wants to make a generic version of a toenail fungus solution made by Valeant Pharma — renamed Bausch Health in May.

But Bausch subsidiary, Valeant Pharmaceuticals North America LLC, has launched a lawsuit against Acrux — and 13 other generic drug developers — to protect its patented drug.

“This action is expected and formally initiates the litigation process under the Hatch-Waxman Act,” Acrux told investors in an ASX announcement on Monday morning

The Hatch-Waxman Act is a set of US legislation that regulates the manufacture of generic drugs.

Acrux and the other developers want to make the active ingredient of the fungus treatment – efinaconazole – as a generic.

Acrux shares plunged on Monday by 24 per cent to a low of 20.5c before recovering to 23.5c by 11am AEST.

There has been conflict as to whether the patent stands since 2016.

Acrux asked for a review of the patent which governs Bausch’s toenail treatment Jublia in 2016, under a rule that allows companies to challenge a patent’s validity or enforceability.

In May last year the US Patent and Trademark Office started the review, and the company said the Office upheld their claims in June this year.

Acrux CEO Michael Kotsanis declined to comment on the company’s initial application for review or grounds for it.

In August, the US Food and Drug Administration (FDA) accepted for review a dossier by Acrux for a generic version of the toenail fungus treatment.

Acrux has to prove “bioequivalence” of its generic to the original in order to be able to make a generic.