“It’s out there on internet forums, but there’s a lot on the internet that’s true or untrue to varying degrees,” he said in an interview with the weekly magazine New Scientist. “It’s a very big deal to have this recorded in a reliable document.”

Others were skeptical about the study and what it could mean for transgender women.

“We all want to have the same experiences as women,” Maria Clifford, 38, a British transgender mother who has been using a surrogate to breast-feed her child, said on Thursday. “We need further testing and evidence to show that these drugs aren’t going to cause harm to the baby.”

“Breast milk is the most important product for the development of a child,” Ms. Clifford added. “We need to make sure it’s pure and hormone free.”

The transgender woman in the experiment — she, her partner and their hometowns were not identified — approached the medical professionals for hormonal medications in 2011 as part of transgender treatment. She had been receiving it for several years before she began breast-feeding, according to the study. She had not had gender-reassignment surgery nor breast augmentation.

She took on the responsibility of breast-feeding because her partner, who was five months pregnant when they approached the hospital, did not want to.

As part of the lactation treatment, the woman stimulated her chest using a breast pump. The study’s authors prescribed progesterone and estradiol, hormones that can influence lactation and that normally occur in pregnant women. They discussed with the couple using domperidone, an anti-nausea drug known to increase breast milk production.

The couple obtained it on their own in Canada, and Ms. Goldstein and Dr. Reisman advised them on the dosage. Though used in many countries, including in Britain, domperidone has been banned in the United States because of Food and Drug Administration concerns about its associations with cardiac arrest and sudden death.