Less than a month after California’s insurance commissioner sued AbbVie, alleging the company used kickbacks to boost sales of Humira, a second class action lawsuit has been filed on behalf of shareholders—and now, a raft of law firms is urging investors to join the fight.

The latest filing, in AbbVie’s home state of Illinois, comes on the heels of a similar suit filed in California. Law firm Pomerantz LLP announced it sued AbbVie on behalf of shareholders who bought the company’s stock between October 2013 and September 2018. The suit stems from the California insurance case, which alleges AbbVie hired nurse “ambassadors” to go to patients’ homes and provide care related to Humira, a drug prescribed to treat rheumatoid arthritis and other autoimmune diseases. The company provided the nurses at no charge to physicians in exchange for Humira prescriptions, the suit alleges.

AbbVie did not immediately respond to a request for comment about the Illinois suit. After the California shareholder case was filed, a spokesperson for the company said AbbVie believes the allegations are “without merit” and that the company’s nurses do not “replace or interfere with interactions between patients and their healthcare providers.”

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RELATED: In kickbacks case, California insurance chief says AbbVie used nurse ‚ambassadors’ to protect Humira sales

California’s action against AbbVie was prompted by a Florida-based whistleblower, a former AbbVie nurse who alleged the company used several tactics to keep patient complaints about Humira from reaching physicians. The state’s insurance commissioner also alleges the company paid out cash, meals and other incentives to prescribers of the drug.

News about the California claim caused AbbVie’s stock to fall 4.5% over two trading days to $91.02, the Illinois suit states. It alleges that during the five years covered by the complaint, AbbVie failed to disclose that its “strategy to increase the sales growth of its blockbuster drug, Humira, relied in part upon illegal kickbacks and unlawful sales and marketing tactics,” according to a statement.

As an $18 billion-a-year blockbuster, Humira is not only the world’s best-selling drug, it also accounts for two-thirds of AbbVie’s total revenues.

This is far from the first time the company has been called out for various strategies it has used to protect its most valuable asset. A recent report found 89% of the patents on Humira were filed after the FDA approved it, for example. AbbVie’s aggressive defense of the drug has sparked lawsuits from the likes of Boehringer Ingelheim, a would-be biosimilar competitor that’s taking the drug’s innovator to task for filing overlapping or non-inventive patents on the product.

RELATED: AbbVie’s not the only one facing kickbacks scrutiny. Add Biogen, Sanofi, Gilead and more

So far, AbbVie is winning the patent battle. Although biosimilar competition is hitting the European market now, the company has struck settlements that will keep cheaper rivals off the U.S. market until 2023.

But now AbbVie has to grapple with aggrieved shareholders. In the past day alone, at least three law firms representing investors have sent alerts to their clients, urging them to join the class action lawsuits that have been filed against the company.