Sigma Pharmaceuticals Ltd is considering an updated acquisition offer of 55 cents cash per share, from South African bidder Aspen Pharmacare Holdings Ltd.

In a statement to the Australian Securities Exchange, Sigma advised its shareholders to take no action on the new bid worth $648 million, saying it will make a further declaration on the offer at the proper time.

Aspen's revised offer is eight per cent lower than the original bid of 60 cents. It is still subject to a number of conditions, including continued due diligence, the extension of recently-expired exclusivity arrangements (to August 2) and regulatory approvals.

A contract would also require shareholder and court approvals for a scheme of arrangement.

Recent trading for Sigma priced shares at 39.5 cents, setting the revised offer at a 39 per cent premium. The first bid was 71 per cent higher than Sigma's share price the day before it was declared on May 21.

The pharmaceutical company's shares have been beaten this year following a heavy writedown on its generic drugs business and resignations from its chairman and chief executive.