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What is it used for?
Tolazoline HCI is an intravenous injectable alpha-agonist reversal agent. The solution itself is to help reverse the sedative effects of substances such as xylazine. Reversing the effects of analgesia and sedation is necessary for a variety of procedures in animals that require the depression of the central nervous system, or CNS. In veterinary practice, it is most commonly utilized with horses.

Side Effects
There are a few side effects to remain aware of immediately after injecting Tolazoline into an animal of any size. While most of the side effects that a Tolazoline HCI injection may trigger will typically subside anywhere from 60-120 minutes after the initial injection, it is imperative to know what to look for in case of a serious incident or adverse reaction.

Some of the most common side effects that are likely to occur among horses immediately after a Tolazoline HCI injection include:

  • Increase in heart rate (in some instances your animal may also experience episodes of tachycardia)
  • Peripheral vasodilation, which should be monitored at all times if possible. Peripheral vasodilation is measurable when Tolazoline HCI is injected in or around the conjunctiva, the gingiva, and even the mucous membranes themselves.
  • Hyperalgesia is common in some cases, though it is not always present. Hyperalgesia will involve an increase in the flicking and licking of your horse’s lips.
  • Muscle twitches and fasciculations
  • Agitation and/or mood swings may be observed in addition to mild apprehension once the drug is administered

These side effects are not extremely uncommon and should wear off anywhere from one to two hours (60-120 minutes) after you administer the initial dosage of the Tolazoline itself.

It is imperative to keep a close eye on any animal injected with Tolazoline for potential heart issues, breathing trouble, or other serious side effects that warrant contact with a veterinarian or emergency veterinarian’s office immediately.

Precautions with this medication

It is important to note that Tolazoline HCI has not been tested nor evaluated in horses with the following conditions:

  • Extreme stress and anxiety
  • Agitation
  • Debilitation
  • Hypovolemia
  • Shock
  • Cardiac disease
  • Blockages (sympathetic)

Tolazoline HCI has not been tested and evaluated on horses that have metabolic conditions or diseases, especially metabolic conditions that are classified as unstable. Additionally, Tolazoline HCI has not been evaluated among horses that are pregnant, nor on horses that are intended for breeding along with lactating male horses.

Overall, the safety and efficacy of a Tolazoline HCI injection have not been studied as a pre-anesthetic to a general anesthetic for the purpose of reversing the overall anesthetic and sedative effects of xylazine.

Because of the anti-sedative and anti-analgesic effects of a Tolazoline HCI, it is important to keep in mind that any animal that is administered the drug may return to a normal perception of pain. Tolazoline HCI should always be measured and administered properly and at the correct rate to prevent potential adverse reactions and/or not enough relief.

This medication should be used with caution on animals who:

  • Have an already diagnosed metabolic condition that is unstable or unable to be controlled. 
  • Have a pre-existing cardiac condition or disease
  • Have metabolic issues such as extreme heart palpitations or undiagnosed cardiac-centric issues
  • Have been diagnosed with ischemic heart disease
  • Have been diagnosed with extreme hypertension or high blood pressure

Safety with Tolazoline HCI
Whenever you are using Tolazoline HCI for sedative and analgesic purposes, it is important to note that there may be gastrointestinal motility issues and/or increased flatulence after the administration of the substance itself. While most gastrointestinal disturbances are mild and occur within just a few minutes of the administration of Tolazoline, it is also possible for a horse to experience diarrhea as well mild colic in some scenarios.

Ensuring clean and sanitary surroundings is highly advisable for anyone administering Tolazoline HCI with horses, regardless of the number of doses required as well as the size of the necessary dose.

Heart rates may briefly increase after the initial injection of Tolazoline HCI, but should come back to a normal resting heart rate anywhere from 5 to 10 minutes after the administering of the injection is complete.

As Tolazoline HCI was tested and monitored at higher doses in horses who were otherwise healthy and free from metabolic conditions, intraventricular conduction was observed. In extremely high doses, intraventricular conduction can lead to serious and life-threatening scenarios.

Possible drug and medication interactions
There are a few cautions to consider along with possible medication interactions that may occur with the use of Tolazoline HCl. Using the following substances and/or medications in conjunction with Tolazoline HCl can lead to serious, if not life-threatening side effects:

  • Dopamine: Dopamine has the ability to trigger severe hypotension when used in conjunction with Tolazoline HCI
  • Norepinephrine, as well as Epinephrine, can quickly lead to an increase in blood pressure and/or cause a paradoxical drop in blood pressure 

How long does it take to work?

Because Tolazoline HCI is administered as an injectable solution, the onset of the Tolazoline itself is generally within 5 minutes or less. The onset of Tolazoline HCI, though typically 5 minutes, may vary depending on the amount administered, the weight of the horse, as well as the depth of the injection site.


When using Tolazoline HCI, it is important to do so as a licensed and registered veterinarian. The drug Tolazoline HCI is not available over-the-counter or to individual pet owners. Tolazoline HCI should only be handled and injected by a professional veterinarian who is well-equipped and familiar with the process of preparing and administering the solution.

Prior to administering the first dosage of Tolazoline HCI, the veterinarian should determine whether or not there are additional safety precautions to take based on your horse’s current health, pre-existing conditions, and whether or not the horse is exhibiting stress or agitation. Your horse’s weight should be calculated into the overall dosage of the Tolazoline HCI solution.

Dosage for Horses

The dosage of Tolazoline HCI for horses will vary based on your horse’s specific condition and/or need as well as your horse’s overall health and weight during the time of surgery or sedation.

Typically, Tolazoline HCI is administered to horses based on body weight. Tolazoline should be given as 4.0 mg per KG of body weight or 1.8 mg per LB of body weight to assist with the process of reversing the analgesic and sedative effects of xylazine, a common non-narcotic sedative compound.

When administering Tolazoline HCI, it should be done slowly, at a rate of approximately 1 mL/second to avoid overdosing or triggering potential adverse side effects. Correct dosages are as follows for horses based on body weight:

Horse weight: 250 KG/550 LB — Tolazine Injection Volume: 10 mL
Horse weight: 500 KG/1100 LB — Tolazine Injection Volume: 20 mL

Using the reference guide of 4 mL/100 kg or 4 mL/220 lb can help you to quickly determine the proper dose of Tolazoline HCI for your horse.

What if I miss a dose?

Missing a dose of Tolazoline HCI is extremely uncommon, as Tolazoline is typically administered in real-time prior to/or during a surgery that requires sedation. However, if the veterinarian or professional administering the Tolazoline HCI does not provide enough of the medication or forgets to administer it, it is possible to still administer the necessary amount.

Signs and Symptoms of an Overdose

While it is extremely common when using Tolazoline HCI on horses for anti-sedative purposes, it is still possible to experience adverse reactions. Understanding the signs and symptoms of a serious potential overdose when using Tolazoline HCI can help you to prevent them and to seek proper remedies and solutions if your horse experiences a negative reaction themselves. Serious signs and symptoms of any overdose when administering Tolazoline HCI may include:

  • Extremely rapid heart palpitations and/or struggles with chest area
  • Hypotension or severely high blood pressure
  • Nausea/vomiting and other gastrointestinal symptoms should be treated as serious and/or life-threatening and should be addressed immediately
  • Heavy breathing, panting
  • Difficulty breathing and/or other automatic response effects

If you believe your horse is experiencing an adverse effect after receiving an injection of Tolazoline HCI solution, inform your veterinarian immediately. In most cases, the common side effects and symptoms that are most likely to occur among animals after receiving a Tolazoline HCI injection will dissipate anywhere from 60 to 120 minutes. 


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