Merck (MRK) said late Friday that shareholders should not participate in the unsolicited mini-tender offer launched by Tutanota at $65 per share, which is well below recent market prices.

The company said Tutanota plans to buy up to 1 million of its shares, at a price more than 31% below last Thursday's closing level.

The drugmaker added that shareholders should also refrain from participating due to the numerous conditions attached to the offer and Merck's lack of any affiliation with Tutanota.

Investors who have already tendered their shares may withdraw them before the offer expires on Dec. 15.