Oestrogens used in the HRT drugs Prempak C and Premarin, manufactured by Wyeth-Ayerst, are extracted from the urine of pregnant horses. More than 45,000 of the animals are farmed each year, mainly in the Canadian provinces of Alberta, Manitoba and Saskatchewan.
The hormonal complex of 10 oestrogens, used to counter menopausal symptoms, can be extracted only when the mares are in foal.
Welfare campaigners are concerned because the horses are confined to stalls during pregnancy, which allows the urine to be collected but prevents them from exercising adequately. They are also concerned that some of the foals may be used as pet food.
Wyeth-Ayerst is the only manufacturer to extract oestrogens from mares' urine. Other manufacturers of HRT drugs use synthetic hormones.
Vicky Hollier, spokeswoman for Wyeth-Ayerst, said the firm takes animal welfare seriously.
"Mares are not confined to stalls throughout their lives. They are regularly exercised and are kept out of doors at pasture, apart from during severe winter months when temperatures can reach minus 40 degrees.
"To protect the horses from the weather conditions, ranchers in these areas usually keep the animals indoors. This is one of the reasons why only Canadian and North American ranches are used in urine collection."
Wyeth-Ayerst buys urine, at £10 per gallon, from about 480 farms in Canada and North Dakota in the US, and employs eight inspectors to police the farms.
But investigators from People for the Ethical Treatment of Animals (PETA), claim to have visited a number of farms where pregnant mares were tethered for long periods without bedding or exercise - a clear breach of the company's code of practice.
PETA is now planning to mount a consumer boycott of Wyeth-Ayerst's products if the company does not use alternative sources of the hormone.
"We're always getting calls from women who are concerned that there's no alternative to Wyeth's products," said Amanda Bates, spokeswoman for PETA. "There are alternatives.
"We're not against HRT, we're against the confinement of pregnant mares for long periods for no satisfactory reason."
Rival drug companies are now concerned about the impact a consumer boycott would have on their synthetic HRT drugs. Schering Healthcare, with several other drug companies, wrote to GPs across the country last week to stress that their products do not contain animal-based oestrogen.
A spokesman for Schering Healthcare said: "Since all this started, we have been getting calls from doctors and patients about the source of Schering's HRT. We felt we were entitled to put our position on this, as many women do not understand it."
Maggie Jennings, head of research for the RSPCA and who flies to Winnipeg today, said she was keeping an "open mind" about the controversy: "We are concerned about the quality of life of the horses.
"If they are keeping them for six months of the year without exercise, then that is a very sad quality of life."
Vets and experts in horse behaviour will be joining Ms Jennings on the week-long visit.Reuse content