"As new CEO Laxman Narasimhan steps in (effective Sept. 1), this clarifies the legal environment for RB and should allow the new management to focus on the RB 2.0 transformation," JP Morgan analysts wrote in a note.
In an effort to regain investor confidence after setbacks including a safety scandal in South Korea, a failed product launch and a cyber attack, RB's outgoing boss Rakesh Kapoor launched a plan to split the group into two business units – one for health and one for hygiene and home products.
But investors feared the U.S. investigations could hinder the company's transformation.
"(The deal) gives the new CEO one less problem to worry about," Hargreaves Lansdown senior analyst Laith Khalaf said.
However, the settlement marks the largest payout so far by any drug company involved in what U.S. President Donald Trump has called the country's epidemic of opioid addiction. The next largest one was for $600 million and was paid in 2007 by Purdue Pharma, the maker of the blockbuster opioid drug, OxyContin.
While welcoming the settlement, AJ Bell investment director Russ Mould said the cost could limit new CEO Narasimhan's ability to undertaken much-needed investment in RB's brands.
"The danger is that Narasimhan will be operating with one hand tied behind his back, he said.
RB said its agreements with the U.S. Department of Justice and the Federal Trade Commission resolved investigations into a business that it demerged in 2014.
Indivior has been charged in the United States with illegally increasing prescriptions for opioid addiction treatment Suboxone by deceiving doctors and healthcare benefit programmes into believing its film version was safer and less susceptible to abuse than similar drugs.
Suboxone is itself a form of opioid.
The indictment said Indivior's wrongdoing began before it was spun out of RB, although RB was not charged.
"While RB has acted lawfully at all times and expressly denies all allegations that it engaged in any wrongful conduct, after careful consideration, the board of RB determined that the agreement is in the best interests of the company and its shareholders," RB said in a statement.
Every day, more than 130 people die in the United States after overdosing on opioids, according to the National Institute on Drug Abuse. Drug overdose deaths rose from 8,048 in 1999 to 47,600 in 2017.
RB said it would increase its provision related to the investigations to $1.5 billion to cover both the cost of the settlement and "any remaining litigation exposures."
The company said the settlement would be funded through existing borrowing facilities and cash generation.
Separately on Thursday, Indivior raised its full-year profit and revenue guidance after Suboxone lost market share at a slower pace than expected, sending its shares 35% higher.
($1 = 0.7981 pounds)(Additional reporting by Siddharth Cavale in Bengaluru; Editing by Edmund Blair and Mark Potter) ((firstname.lastname@example.org; Within UK +44 20 7542 1810; Outside UK +91 80 6749 2764; Reuters Messaging: email@example.com))