WASHINGTON (Reuters) - Dr. Margaret Hamburg, who as commissioner of the U.S. Food and Drug Administration (FDA) for almost six years has overseen public health initiatives ranging from tobacco control and food safety to personalized medicine and drug approvals, is stepping down, the agency said on Thursday.

Margaret Hamburg, Commissioner of the U.S. Food and Drug Administration (FDA), speaks during the 2013 Reuters Health Summit in New York, in this May 6, 2013 file photo. REUTERS/Shannon Stapleton/Files

Hamburg, one of the longest-serving FDA commissioners in the modern era, told Reuters in an interview that her decision was prompted by the heavy demands of the job and the sheer length of time she has held the position.

“This is a very challenging job full of opportunities to make a huge and enduring difference,” she said, “but it is 24/7 and there are really really difficult decisions to make.”

The 59-year-old, nominated by President Barack Obama and confirmed by the U.S. Senate in May 2009, was last year named the world’s 51st most powerful woman by Forbes magazine.

Hamburg’s resignation comes at a crucial time for the FDA as Congress pushes initiatives to speed new drug development, and food safety advocates, backed by Obama, back the creation of a separate agency combining the food safety functions of the FDA and the U.S. Department of Agriculture.

Under Hamburg, the agency introduced multiple measures to speed new products to the market. In 2014, the FDA approved 51 new therapies, the most in almost 20 years. In a blog post on Wednesday, Hamburg called the achievement “a testament ... to FDA’s innovative approaches to help expedite development and review of medical products that target unmet medical needs.”

A spokesman for the White House, Josh Earnest, told reporters at a briefing that Obama’s nomination to replace Hamburg would have impeccable scientific credentials and merit bipartisan support. The nominee must be confirmed by the Republican-controlled Senate.

Late last month, the agency named Dr. Robert Califf, a prominent cardiologist and researcher from Duke University, to oversee its drug, medical device and tobacco policy. Califf is viewed by many as a potential successor to Hamburg, whose resignation takes effect in March.

Dr. Stephen Ostroff, the FDA’s chief scientist, will fill Hamburg’s position until a new commissioner is named.

CAREER IN PUBLIC HEALTH

A long-time public health official with extensive experience fighting AIDS and tuberculosis, Hamburg, who graduated from Harvard Medical School, previously served at the National Institutes of Health before becoming New York City’s health commissioner. That public health focus endeared her to patient advocates.

“Commissioner Hamburg, from day one, has been committed to being a champion for patients,” said Ellen Sigal, founder and chair of Friends of Cancer Research. “She has fostered the growth of science and innovation across the agency and really changed how FDA and industry collaborate.”

Under Hamburg, the FDA, which oversees products representing more than 20 cents of every dollar spent by U.S. consumers, has proposed measures to improve nutrition by limiting dangerous trans-fats in food and requiring restaurants to post calorie counts on menus. It also has beefed up inspections of food and drugs from overseas.

“She really had an active and visible role on the global stage not only from a medical products perspective but from a food safety perspective,” said Erica Jefferson, former acting assistant commissioner for media affairs.

During her tenure the FDA has confronted major public health issues, including the rise of antibiotic-resistant bacteria, the abuse of opioid painkillers, the emergence of electronic cigarettes and the outbreak of Ebola and other infectious diseases.

Her ride has not always been smooth. She faced hostile questioning by Republicans in Congress following a fungal meningitis outbreak in 2012 that killed dozens of people and sickened hundreds more.

In 2011, then Health and Human Services Secretary Kathleen Sebelius overruled the FDA’s decision to allow an emergency contraceptive known as Plan B to be sold over the counter to young teenagers. Hamburg insisted Plan B was safe for use and it was approved two years later.

Hamburg, whose mother was the first African-American woman to earn a degree from Yale University School of Medicine, was never a crusading commissioner in the way of one of her predecessors, Dr. David Kessler, who fought to bring tobacco under FDA regulation. The FDA eventually gained authority over tobacco in 2009.

Hamburg’s tenure has disappointed some drug watchdogs, who say the FDA has too often succumbed to industry pressure.

“Throughout Hamburg’s tenure, the FDA has grown even more cozy with the industries that it regulates,” Public Citizen said in a statement.

Yet it was Hamburg who, in 2011, revoked approval for Roche Holding AG'sROG.VX drug Avastin as a treatment for metastatic breast cancer after the company failed to prove that the benefits outweighed the risks. The drug remains on the market for certain types of colon, lung, kidney and brain cancer.

And she began a crackdown on poor-quality generic drugs and drug ingredients from India, China and elsewhere.