Sigma said its earnings would fall $5 million to $10 million a year if the My Chemist group were allowed to buy certain products from a rival wholesaler.

Sigma, which also owns the Amcal and Guardian retail brands, filed a writ in the Supreme Court of Victoria on Wednesday against Administration and Marketing Solutions, which owns Chemist Warehouse, My Chemist, E Pharmacy and other brands.

In it Sigma says a supply agreement signed in February 2016 prevents the My Chemist group from seeking or buying goods that Sigma sells from a different wholesaler.

Despite this, the My Chemist group's co-founder, Mario Verrocchi, emailed Sigma chief executive Mark Hooper in February telling him the group had received an offer from a rival wholesaler that the company "cannot ignore because of its quantum", the writ says.

Sigma says Mr Verrocchi then told Mr Hooper on May 5 that the company would consider itself free of the supply contract unless Sigma matched a deal the rival wholesaler had offered for Sandoz generic pharmaceuticals.

"Offers of mediation and binding arbitration were also proposed but were rejected by [My Chemist]," Sigma said.

"As a satisfactory resolution has not been reached, Sigma has now decided to commence legal proceedings."

Sigma is seeking declarations and injunctions to stop the chemist group from buying the products from other wholesalers until the end of 2018.

Sigma said the dispute and court action, combined with a subdued start to the financial year, could mean earnings before interest and tax might fall as much as 5 per cent this year compared with 2016, when EBIT was 100.9 million.

Sigma's shares had fallen 31 per cent by 3.30pm on Wednesday to 81?? - their lowest point since February 2016.

The court action also names Mr Verrocchi and the company's fellow co-founders, brothers Jack and Sam Gance, as defendants.