V. Craig Jordan, a pharmacologist whose discovery that a failed contraceptive, tamoxifen, could block the growth of breast cancer cells opened up a whole new class of drugs and helped save the lives of millions of women, died on June 9 at his home in Houston. He was 76.

Balkees Abderrahman, a researcher who worked closely with Dr. Jordan and was his caregiver for several years, said the cause was renal cancer.

Dr. Jordan was known as a meticulous, even obsessive researcher, a quality demonstrated in his work on tamoxifen. The drug was first synthesized in 1962, though it was discarded after not only failing to prevent conception but, in some cases, promoting it.

But Dr. Jordan, then still a doctoral student at the University of Leeds in Britain, saw something that no one else did. It had long been known that estrogen promoted breast cancer growth in postmenopausal women — and he suspected that tamoxifen could help stop it.

Cancer of all kinds had long been seen as an unconquerable foe, treatable only with blunt, dangerous tools like chemotherapy. But the early 1970s saw a new wave of research, fueled in part by President Richard M. Nixon’s “war on cancer” campaign, which over the next 30 years would lead to a revolution in oncology.

Dr. Jordan was a leader in that revolution. Over decades of research, he was able to show that tamoxifen, when given to patients with early-stage breast cancer, interrupted the tumor’s growth by blocking its estrogen receptors. It was, in his words, “anti-estrogen.”

Approved by the Food and Drug Administration in 1999, tamoxifen was the first in a new class of drugs called selective estrogen receptor modulators. It and other drugs are now prescribed to women around the world, and are credited with helping millions of patients.

Tamoxifen isn’t perfect. It works on 65 percent to 80 percent of postmenopausal patients, and just 45 percent to 60 percent of premenopausal patients. And Dr. Jordan was the first to reveal that it led to a small increase in the risk of a kind of uterine cancer — though he argued that the benefits for breast cancer patients were still overwhelming.

In 1998 Dr. Jordan, working with Steven R. Cummings, an expert on aging at the University of California, San Francisco, showed that another estrogen-blocking drug, raloxifene, both improved bone density in postmenopausal women and reduced their risk of developing breast cancer by as much as 70 percent.

Dr. Jordan was in many ways an old-school researcher. He insisted that a drug should be investigated for all its potential applications, not just the ones that might make money or be the quickest to market. And he believed that scientists should be transparent about side effects, even if it meant reducing a drug’s appeal. He called his work “conversations with nature.”

Virgil Craig Jordan was born on July 25, 1947, in New Braunfels, Texas. His British mother, Cynthia Mottram, and his American father, Virgil Johnson, had met while his father was serving in England during World War II and then returned to his home in Texas after the war.

They divorced soon after Craig was born, and he and his mother moved to her home in Bramhall, near Manchester, where he grew up. She later married Geoffrey Jordan, who adopted Craig as his son.

By his own account, Craig was a mediocre student. The only subject in which he excelled was chemistry, a passion that his mother fostered by letting him build a laboratory in his bedroom.

“Experiments would often get out of hand, so a fuming brew would be hurled out of the window onto the lawn below, leaving the curtains ablaze,” he wrote in Endocrine Journal in 2014. “Naturally, the lawn died.”

Given his poor grades, he assumed that he would go straight from high school to the work force, perhaps as a lab technician at a nearby plant run by Imperial Chemical Industries (which today is part of the pharmaceutical giant AstraZeneca).

But his mother leaned on his teachers to give him another year of study to prepare for college, and he managed to win a scholarship to the University of Leeds. He earned a bachelor’s degree in 1969, a Ph.D. in 1973 and a doctorate of science in 1985, all in pharmacology.

He also joined the University Officers’ Training Corps, after which he served in the British Army and its reserves until mandatory retirement at 55 — most of the time with the elite Special Air Service, a rough equivalent to the U.S. Navy SEALs.

While at Leeds, he began working on tamoxifen, an interest that he took with him through a series of positions at several institutions: the Worcester Foundation for Experimental Biology in Shrewsbury, Mass.; the University of Wisconsin; Northwestern University; the Fox Chase Cancer Center in Philadelphia; Georgetown University; and, starting in 2014, the MD Anderson Cancer Center at the University of Texas in Houston.

Dr. Jordan’s three marriages ended in divorce. He is survived by two daughters from his first marriage, Alexandra Noel and Helen Turner, and five grandchildren.

He was diagnosed with Stage 4 renal cancer in 2018, an earth-shattering result that he nevertheless spoke openly about — and that he fought against, and worked through, for the last years of his life.

“I find myself in a state of flux, but I’m not scared of dying,” he told The ASCO Post, an oncology publication, in 2022. “I was the person most likely never to make age 30 with the stupid things I was doing in my youth.”