Generally speaking, dark-money groups are politically active organizations, often nonprofit, that, under I.R.S. regulations, are not required to disclose the identities of their donors.

Such groups are often chartered under Section 501(c)(4) of the tax law, which grants a tax exemption to “social welfare organizations.” For those seeking to influence politics but stay in the background, 501(c)(4) designations offer two big advantages: tax exemption and no requirement to disclose donors.

At a time of high drug prices and its heaviest political expenditures in years, the pharmaceutical industry has directed substantial resources through AAN and other such groups that hide the identity of their donors and have few if any limits on fund-raising.

“PhRMA has always been very aggressive and very effective in their influence efforts,” said Michael Beckel, research manager at Issue One, a nonprofit devoted to campaign-finance transparency. “That includes using these new, dark-money vehicles to influence policy and elections.”

PhRMA’s $6.1 million, unrestricted donation to AAN was its single-biggest grant in 2016, dwarfing its $130,000 contribution to the same group the year before. Closely associated with House Republicans — AAN has a former Republican senator and two former Republican House members on its board — the group backed the failed G.O.P. health bill intended to replace the Affordable Care Act. It also supported the successful Tax Cuts and Jobs Act of 2017, which reduced corporate taxes by hundreds of billions of dollars over a decade.

So far in this election cycle, AAN has given more than $19 million to the Congressional Leadership Fund, a Republican super PAC with which it shares an address and staff, according to the Center for Responsive Politics. The fund recently ran ads opposing Democratic candidates in high-profile special congressional elections in Georgia and Pennsylvania.

PhRMA disputes the suggestion that it backs particular actions by the recipients of its donations. “PhRMA engages with groups and organizations that have a wide array of health care opinions and policy priorities,” said its spokesman, Robert Zirkelbach. “It is inaccurate and would be inappropriate for you to attribute those grants to a specific campaign.”