Shares of Natrol Inc. soared in trading Monday after Indian biotech firm Plethico Pharmaceuticals Ltd. launched a tender offer to buy the supplement company maker for about $81 million which figures out to nearly double the stock’s recent price.

Holders of 42 percent of the shares of the Chatsworth-based company have agreed to tender their shares under the offer, which begins Nov. 27 and lasts 20 business days. Under the deal, shareholders can tender their shares to Plethico for $4.40 per share. If Plethico gains control of more than 90 percent of Natrol’s stock, it will buy the remaining 10 percent at the same price.

At $4.40 per share, outstanding shares in Natrol are worth about $63 million. Plethico also agreed to buy out the holders of Natrol’s stock options by giving them the difference between their exercise price and $4.40. Natrol said that brings the total value of the deal to $80.8 million. The price of $4.40 represents a 97 percent premium on Natrol’s closing price Nov. 16 of $2.27.

Shares in Natrol surged $2.01, or 89 percent, to $4.28 in early trading Monday on the Nasdaq. Shares had gained nearly 7 percent for the year before today.