(Reuters) - Valeant Pharmaceuticals International Inc (>> Valeant Pharmaceuticals Intl Inc) (>> Valeant Pharmaceuticals Intl Inc), target of a securities investigation and under scrutiny for its pricing and accounting practices, said on Monday that its longtime chief executive officer was leaving just three weeks after returning from a medical leave.

The past few months have also put a question mark over the troubled Canadian drugmaker's overall strategy of rapid acquisition-driven expansion, taking on heavy debt and aggressive price hikes.

Following is a summary of key events in Valeant's history:

December 2007: Biovail Corp of Canada, Valeant's predecessor, pays $138 million to settle a shareholder lawsuit accusing it of making false statements to inflate its stock price.

February 2008: California-based Valeant Pharmaceuticals International names McKinsey & Co veteran and pharmaceutical acquisitions expert Michael Pearson as its CEO. It buys Coria Laboratories for $95 million and Australia's DermaTech for $12.6 million.

March 2008: The U.S. Securities and Exchange Commission charges Biovail Corp, its former CEO and three other senior executives with fraudulent accounting and making a series of misstatements to analysts and investors.

January 2009: Valeant buys Dow Pharmaceutical Sciences Inc, a maker of topical dermatology products, for $285 million, and buys Mexican generic drugmaker Tecnofarma.

May 2010: Valeant buys Aton Pharmaceuticals, a New Jersey-based maker of ophthalmology products, for $318 million.

Sept 2010: Valeant is acquired by Biovail in a reverse merger. Pearson becomes CEO of the combined company, with an annual revenue of $1.75 billion. It takes Valeant's name and is incorporated in Canada, taking advantage of a lower corporate tax rate than in the United States.

2011: Valeant settles a civil lawsuit brought by the SEC accusing Biovail of accounting fraud. It boosts its presence in Central and Eastern Europe by buying Switzerland-based generic company PharmaSwiss for $481 million; AB Sanitas of Lithuania for about $500 million; Canada's Afexa Life Sciences and Sanofi SA's dermatology unit Dermik. However, it fails in its $5.7 billion unsolicited bid for U.S. biotech Cephalon to an almost $7 billion offer from Israeli drugmaker Teva Pharmaceutical Industries (>> Teva Pharmaceutical Industries Limited).

Dec. 2012: Valeant buys Medicis Pharmaceutical Corp for $2.6 billion, acquiring anti-wrinkle medicines and facial fillers that compete with Allergan Inc's (>> Allergan, Inc.) market-leading portfolio.

April 2013: Valeant offers more than $13 billion in stock for smaller U.S. rival Actavis Inc, but merger talks collapse.

August 2013: In its biggest deal ever, Valeant buys eye-care company Bausch & Lomb from private equity firm Warburg Pincus for $8.6 billion.

January 2014: After making the list of the world's top 15 drugmakers by market capitalization, Pearson tells analysts Valeant aims to crack the top 5 by the end of 2016.

March 2014: Jim Grant, editor of an investment journal, criticizes Valeant for its lack of concern for research and development.

April 2014: Valeant and activist investor William Ackman's Pershing Square Capital Management hedge fund team up in effort to buy Botox-maker Allergan.

May 2014: Bronte Capital's John Hempton says his fund is shorting Valeant, calling its accounts "difficult to comprehend." James Chanos, founder of Kynikos Associates and short on Valeant, accuses it of "aggressive accounting games."

June 2014: Allergan, fending off Valeant's takeover attempt, releases email exchanges with Morgan Stanley in which the bank called Valeant a "house of cards."

Nov. 2014: Valeant and Ackman end their pursuit for Allergan after rival Actavis outbids them with a $66 billion offer.

March 2015: Pershing Square discloses it has taken a 5 percent stake in Valeant.

April 2015: Valeant completes its $11 billion purchase of Salix Pharmaceuticals, a maker of gastrointestinal medicines.

June 2015: Long-time investor ValueAct Capital Management says it sold 4.2 million Valeant shares, but retains a stake worth over $3 billion.

Sept. 28, 2015: Democratic members of a congressional committee urge chairman to subpoena Valeant over "massive" price increases for two of its heart drugs.

Oct. 15, 2015: Valeant says it has been subpoenaed by U.S. prosecutors seeking details on its patient assistance programs, drug pricing and distribution practices.

Oct. 19, 2015: New York Times reports how Valeant has used its ties with specialty pharmacy Philidor to sell conventional medications, averting health insurer barriers to reimbursement.

In a conference call later that day, Valeant discloses for the first time that it has used Philidor's services, has an option to buy the pharmacy and has already incorporated its financials in its own results.

Oct. 21, 2015: Valeant shares plunge as much as 40 percent after an influential short-seller, Citron Research, accuses the company of using specialty pharmacies, including Philidor, to inflate its revenue. Valeant categorically denies the allegations.

Oct. 26, 2015: Valeant holds investor call to defend itself against Citron allegations and sets up ad-hoc committee to study them in depth. Valeant shares end 5.3 percent lower.

Oct. 30, 2015: Valeant cuts ties with specialty pharmacy distributor, Philidor, accused of helping it inflate revenue. Philidor has since gone out of business. Valeant later warned that its dermatology business would be hurt in the short term.

Dec. 15, 2015: Valeant inks a deal to distribute its drugs through pharmacy chain Walgreens Boots Alliance Inc (>> Walgreens Boots Alliance Inc).

Dec. 16, 2015: Valeant says its Q4 profit was hit when it cut ties with Philidor Rx Services, but it could contain the damage in 2016 and grow profit.

Dec. 28, 2015: Valeant appoints group of company executives to take over duties of Chief Executive Michael Pearson until he returns from medical leave.

Jan. 6, 2016: The company appoints its former CFO, Howard Schiller, as interim CEO.

Jan. 28, 2016: Campaign of Democratic presidential contender Hillary Clinton posts a blog detailing exorbitant price hikes for a migraine drug made by Valeant.

Feb. 4, 2016: At a U.S. congressional hearing, interim CEO Schiller puts forward a conciliatory face, testifying that his company had changed its business and pricing tactics.

Feb. 22, 2016: Valeant says it would restate its financial results for 2014 and 2015 after identifying some sales of Philidor that should have been recognized when products were dispensed to patients.

Feb. 29, 2016: Valeant discloses it is under investigation by the U.S. Securities and Exchange Commission a day after announcing the return of CEO Pearson from medical leave and withdrawing 2016 forecasts.

March 7, 2016: Valeant says it would release preliminary quarterly results and guidance on March 15, two weeks after they were originally scheduled for release.

March 9, 2016: The company adds a representative from shareholder Pershing Square Capital Management to its board, as well as two other new directors.

March 10, 2016: A U.S. congressional committee urges Valeant to explain why it was withholding documents related to an investigation of steep price hikes of two of its heart drugs.

March 15, 2016: Valeant cuts 2016 revenue forecast by about 12 percent and says a delay in filing its annual report could mean a default on its $30 million debt, causing its shares to plunge 50 percent in one day.

March 21, 2016: Valeant says CEO Pearson is leaving and Ackman will join the board as it tries to save its business, and its shares rise more than 7 percent. Valeant blames accounting issues on "improper conduct" by top finance executives, including Schiller, who denies the accusation.

Sources: Reuters, Valeant press statements

(Reporting by Bill Berkrot and Ransdell Pierson in New York and Amrutha Penumundi in Bengaluru)