Premarin is produced from the urine of pregnant mares. However, a fall in sales has seen seven ranches in North Dakota and Alberta, Canada, lose contracts to provide the urine.
Aspen Valley Horse Rescue was formed specifically to save some of the horses and has been able to save 95 animals so far.
Kathy Raife and Heidi Alles, who set up the charity, managed to save 24 horses in the first fortnight after the charity was established, and publicity has since seen another 75 saved.
The exercise is far from cheap. Each horse costs around $US1000 by the time the rancher has been paid for the animal and it has been trucked to Aspen Valley.
Supporters have individually contributed from $2 to $10,000, with a local newspaper reporting that one contributor from as far away as Switzerland gave money to support the effort.
The rescuers are working against time, with a deadline extension having already been granted from October 31 to November 10 to find homes for the horses.
Raife, writing on the rescue's website, said the original expectation was that maybe 10 horses could be rescued.
"We are inundated with emails, phone calls and trying to arrange transportation for all of the horses! Thanks to everyone for their incredible support.
"We could not have done this without help from all of you!"
Another horses rescue, The Animali Farm, is also playing a key role in rehoming efforts.
The group works primarily with ranches moving out of production of mare's urine.