Schering-Plough has added five international firms to its roster of external legal advisers.

Allen & Overy (A&O), Lovells, Eversheds, DLA Piper and Baker & McKenzie, have joined the pharmaceutical giant's legal panel.

The additions, which were made last month, formalise existing relationships with the firms and were not made as part of a wholesale review of Schering's existing, mainly US, legal panel.

A&O and Lovells, for example, both advised Schering on its 2007 €11bn (£9.4bn) acquisition of the Dutch chemicals group Akzo Nobel's medicines business.

Schering's overall legal panel, known within Schering as its 'partner law firm programme', was last reviewed in August 2008 and is due to expire in July next year. It includes firms such as Howrey, Ropes & Gray, Covington & Burling, Reed Smith, Sidley Austin, WilmerHale, Reed Smith, Fox Rothschild and Lowenstein Sandler.

A spokesperson confirmed: "This year, Schering-Plough expanded its programme to include a few additional law firms to further support its diversity initiatives and global outside counsel work."

Schering is involved in a proposed $41bn (£25bn) takeover by US pharmaceutical rival Merck. It is unclear how the merger, which was announced in March but is still awaiting regulatory approval, will affect the company's external legal advisers.

The takeover talks have seen Schering instruct Wall Street leader Wachtell Lipton Rosen & Katz, while Fried Frank Harris Shriver & Jacobson is advising Merck. Wachtell also advised Schering on US aspects of the Akzo Nobel deal.