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Lasix: Two Important Facts Every Horse Trainer Should Know

Updated: May 8, 2023

Fact #1

Lasix should be given IM (intramuscularly). When given in this way the drug is apparently slowly released from the injection site in the muscle into the bloodstream. This is the one way to get around the rapid excretion of Lasix and prolong the effect. Its half-life in the blood is therefore about eighty minutes instead of thirty minutes, and the diuretic effect lasts longer. If one gives a 10ml dose of Lasix by IM injection rather than by IV (intravenously) injection, one gets a 50 percent greater response and the response is spread over three hours instead of one hour after IV administration. IM administration is probably the most effective way to use Lasix.

Fact #2

The effects of Lasix on the heart and blood vessels are also transient, peaking within ten minutes and then declining. After IV injection, Lasix (furosemide) reduces blood pressure in the lung within minutes. This effect occurs independently of the diuretic effect and has been seen in experimental animals in which no urine was being formed. Curiously, however, this effect seems to require furosemide to release prostaglandin from the kidney. Anti-prostaglandin drugs such as Bute (phenylbutazone), therefore, may act to block the pulmonary effects of Lasix, a rather surprising finding in view of the frequency with which these two drugs are used together in racing horses.

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