Furosemide (aka Lasix) has long been a controversial race day drug in U.S. racehorses—not only for its potential performance-enhancing effects but also because studies have shown that it can alter horses’ calcium balance, a mineral that’s important for bone strength and muscle contraction.

“We usually use this term (balance) to refer to intake minus excretion,” said Abby Pritchard, a PhD candidate in Michigan State University’s Department of Animal Science, in East Lansing. “Essentially, we want this to be zero so we know that the horse’s requirements are being met without wasting or creating imbalances with other minerals.”

So Pritchard measured how long it takes a horse’s calcium balance to return to baseline after furosemide administration. She shared her results at the 2019 Equine Science Society Symposium, held June 3-6 in Asheville, North Carolina.

Many trainers have veterinarians administer furosemide (marketed as Salix) to horses on race days to prevent exercise-induced pulmonary hemorrhage (EIPH, bleeding of the blood vessels in the lungs). Because it’s a diuretic—increasing urination—it can cause a horse to drop weight rapidly, a potential performance-enhancing side effect. Increased urine output also translates to increased sodium and calcium losses.

Previous research has documented that furosemide negatively affects calcium balance three days post-administration but did not determine when calcium balance returned to baseline,” said Pritchard.

In her crossover study she compared calcium levels in five mature Standardbreds receiving furosemide and five not receiving the drug (controls). The treatment horses received one standard dose of the medication, after which Pritchard collected and measured both groups’ urine and manure output for seven days. After a two-week washout period, she swapped treatment and control group horses and repeated the experiment.

Based on her results Pritchard found:

  • No difference in horses’ fecal output or fecal calcium levels;
  • Treated horses urinated more than twice as much as control horses on Day 1, then the frequency dropped back to normal;
  • Treated horses’ urine calcium levels were twice as high as controls on Day 1, but they returned to normal by Day 3; and
  • Any negative effects of furosemide on calcium balance were almost gone by Day 3 and disappeared completely by Day 5, making them no different than controls.

“Most of the calcium balance effects seen are driven by urine output and calcium loss on Day 1,” said Pritchard.

These findings—that calcium levels return to baseline within five days—suggest that weekly furosemide administration in racehorses doesn’t lead to long-term calcium losses that might contribute to skeletal injuries, she said.