In addition, the email said GSK had engaged in apparent Syrian export control violations, including an alleged smuggling scheme to ship the drug component pseudoephedrine to Iran from Syria via Iraq. Pseudoephedrine is regulated as a precursor for making methamphetamine.
GSK said it would investigate this matter along with the bribery claims.
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"We welcome people speaking up if they have concerns about alleged misconduct," the company said.
"On 18 July 2014, we received an email making claims regarding GSK's former consumer operations and related distributors in Syria. Our compliance and legal departments were immediately notified and, as is our standard procedure, we immediately responded to the sender to confirm receipt and ask for more information."
The whistleblower's email said GSK used its own employees and Syrian distributor Maatouk Group to make illicit payments.
An official at Damascus-based Maatouk had no comment when contacted by telephone and said the company's top executives were not immediately available.
The email listed a range of alleged improper activities, including payments of $1,500 each to two doctors to promote Panadol. The document also highlighted bribes paid to pharmacists and payments for medics to visit a Mediterranean holiday resort.
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Further cash payments were related to the promotion of GSK cold and flu products, as well as its premium toothpaste brand Sensodyne.
Bribery charges around the world have tarnished the reputation of Witty and hit the company's sales in China, at a time when it is also struggling with sluggish sales growth in the all-important U.S. market.
The allegations also leave it open to legal action - and potentially hefty fines - in Western countries where it is based or has a stock market listing.
Britain's Serious Fraud Office launched a formal criminal investigation into GSK's overseas activities in May and the U.S. Department of Justice (DOJ) is investigating it for possible breaches of the Foreign Corrupt Practices Act (FCPA).
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In the email sent to GSK concerning Syria, the author said that the information would be passed on to the DOJ and the U.S. Securities and Exchange Commission (SEC).
A recently introduced SEC program provides cash incentives for whistleblowers to report corporate malpractice, including breaches of the FCPA.
GSK has overhauled its marketing policies in the wake of concerns about possible past misconduct. It aims to become the first company in the industry to stop paying outside doctors to promote its products.
- By Reuters