Xylazine

Xylazine is often used as a sedative, muscle relaxant, and analgesic.[1] It is frequently used in the treatment of tetanus.[1] Xylazine is very similar to drugs such as phenothiazine, tricyclic antidepressants, and clonidine.[4] As an anesthetic, it is typically used in conjunction with ketamine.[10] Xylazine appears to reduce sensitivity to insulin and glucose uptake in humans.[10] Yohimbine has been used to decrease glucose levels to a healthy level.[10] In clinical settings, Yohimbine can reverse the adverse effects of xylazine if administered intravenously at a dosage of 0.5 mL / 20 pounds shortly after xylazine administration.[8]

Xylazine overdose is usually fatal in humans.[1] Because it is used as a drug adulterant, the symptoms caused by the drugs accompanying xylazine administration vary between individuals.[9]

The most common side effects in humans associated with xylazine administration include bradycardia, respiratory depression, hypotension, transient hypertension secondary to vagus nerve stimulation, and other changes in cardiac output.[1][9] Xylazine significantly decreases heart rate in animals that are not premedicated with medications that have anticholinergic effects.[1] The decrease in heart rate directly impacts aortic flow.[1] Bradycardia caused by xylazine administration is effectively prevented by administration of atropine or glycopyrrolate.[1] Arrhythmias associated with xylazine includes other symptoms such as sinoatrial block, atrioventricular block, A-V dissociation, and sinus arrhythmia.[1]

Xylazine administration can lead to diabetes mellitus and hyperglycemia.[10] Other possible side effects that can occur are areflexia, asthenia, ataxia, blurred vision, disorientation, dizziness, drowsiness, dysarthria, dysmetria, fainting, hyporeflexia, slurred speech, somnolence, staggering, coma, apnea, shallow breathing, sleepiness, premature ventricular contraction, tachycardia, miosis, and dry mouth.[4] Rarely, hypotonia, dry mouth, urinary incontinence and nonspecific electrocardiographic ST segment changes occur.[4] It has been reported that the duration of symptoms after human overdose is 8 to 72 hours.[4] Further research is necessary to categorize the side effects that occur when xylazine is used in conjunction with heroin and cocaine.[1]

Chronic use is reported to be associated with physical deterioration, dependence, abscesses, and skin ulceration, which can be physically debilitating and painful.[4][10] Hypertension followed by hypotension, bradycardia, and respiratory depression lower tissue oxygenation in the skin.[9] Thus, chronic use of xylazine can progress the skin oxygenation deficit, leading to severe skin ulceration.[9] Lower skin oxygenation is associated with impaired healing of wounds and a higher chance of infection.[9] The ulcers may have a characteristic odor and ooze pus.[8] In severe cases, amputations must be performed on the affected extremities.[8]

The known doses of xylazine that produce toxicity and fatality in humans vary from 40 to 2400 mg.[4] Small doses may produce toxicity and larger doses may be survived with medical assistance.[4] Non-fatal blood or plasma concentration ranges from 0.03 to 4.6 mg/L.[11] In fatalities, the blood concentration of xylazine ranges from trace to 16 mg/L.[11] It is reported that there is no defined safe or fatal concentration of xylazine because of the significant overlap between the non-fatal and postmortem blood concentrations of xylazine.[4]

Currently, there is no specific antidote to treat humans that overdose on xylazine. Hemodialysis has been suggested as a form of treatment, but is usually unfavorable due to the large volume of distribution of xylazine.[4] In addition, due to lack of research in humans, there are no standardized screenings to determine if an overdose has occurred. The detection of xylazine in biological fluids in humans involves various screening methods, such as urine screenings, thin layer chromatography (TLC), gas chromatography mass spectrometry (GC-MS), Remedi HS Bio-Rad Laboratories, and liquid chromatography mass spectrometry (LC-MS).[12][11]

Multiple drugs have been used as supportive therapeutic intervention such as lidocaine, naloxone, thiamine, lorazepam, vecuronium, etomidate, propofol, tolazoline, yohimbine, atropine, orciprenaline, metoclopramide, ranitidine, metoprolol, enoxaparin, flucloxacillin, insulin, and irrigation of both eyes with saline.[4] Effects of xylazine are also reversed by the analeptics 4-aminopyridine, doxapram, and caffeine, which are physiological antagonists to central nervous system depressants.[13] Combining yohimbine and 4-aminopyridine in an effort to antagonize xylazine is superior as compared to the administration of either of these drugs individually due to reduction of recovery time.[13] Research initiatives may be necessary in order to standardize treatment and determine effective measures for identifying chronic xylazine usage and intoxication.[1]

The treatment after xylazine overdose should primarily involve maintaining respiratory function and blood pressure.[4] In cases of intoxication, physicians recommend intravenous fluid infusion, atropine, and hospital observation.[1] Severe cases may require endotracheal intubation, mechanical ventilation, gastric lavage, activated charcoal, bladder catheterization, electrocardiographic (ECG) and hyperglycemia monitoring.[4] Physicians typically recommend which detoxification treatment should be used to manage possible dysfunction involving highly perfused organs, such as the liver and kidney.[11]

Xylazine was discovered as an antihypertensive agent in 1962 by Farbenfabriken Bayer in Leverkusen, Germany.[1] Results from early human clinical studies confirmed that xylazine has several central nervous system depressant effects.[1] Xylazine administration is used for sedation, anesthesia, muscle relaxation, and analgesia.[2] It causes a significant reduction in blood pressure and heart rate in healthy volunteers.[10] Due to hazardous side effects, including hypotension and bradycardia, xylazine was not approved by the Food and Drug Administration (FDA) for human use.[9] As a result, xylazine's mechanism of action in humans remains unknown.[4]

Xylazine was approved by the FDA for veterinary use and is now used as an animal tranquilizer.[9] In the United States, xylazine was only approved by the FDA for veterinary use as a sedative, analgesic, and muscle relaxant in dogs, cats, horses, elk, fallow deer, mule deer, sika deer, and white-tailed deer.[1][4] The sedative and analgesic effects of xylazine are related to central nervous system depression. Xylazine's muscle relaxant effect inhibits the transmission of neural impulses in the central nervous system.[15]

In scientific research, xylazine is a component of the most common anesthetic, ketamine-xylazine, which is used in rats, mice, hamster, and guinea pigs.[17] The accounts of the actions and uses of xylazine in animals were reported as early as the late 1960s and early 1970s.[1] Since the early 2000s, xylazine has become popular as a drug of abuse in the United States and Puerto Rico.[12] Xylazine's street name in Puerto Rico is Anestesia de Caballo, which roughly translates to “horse anesthetic."[4][8] The reasons that may explain why this drug has become increasingly popular are still unknown.[8] Further research is needed to gain more information on the distribution of xylazine in the body, physical symptoms, potential treatments, and predictive factors for chronic usage.[8]

Xylazine users are more likely to be male, under the age of 30, living in a rural area, and injecting versus inhaling xylazine.[9] Xylazine has similar behavioral consequences as heroin, thus it is commonly used as an adulterant.[8] Xylazine is also frequently found in speedball.[4] The combination of heroin and xylazine produces a stronger high than administration of heroin alone.[4] Concomitant use of xylazine with speedball may potentiate or prolong the effects of these drugs, which can lead to adverse consequences.[11]

Carruthers et al. in 1979 reported the first case of xylazine toxicity in a 34-year-old male, who self-medicated for insomnia with injection of 1 g of xylazine.[18] Intentional intoxication from ingesting, inhaling, or injecting xylazine has been reported.[9] The intravenous route is the most common route of administration for those who abuse heroin or xylazine recreationally.[11] In Puerto Rico, xylazine has increased in popularity. Its use was associated with a high number of inmate deaths at the Guerrero Correctional Institution in Aguadilla, Puerto Rico from 2002 through 2008.[9]

When used as a drug of abuse in humans, the frequency of dosage is dependent on social or economic factors as well as each user's subjective response to xylazine's addictive properties.[1]

In animals, xylazine may be administered intramuscularly or intravenously.[4] As a veterinary anesthetic, xylazine is typically only administered once for intended effect before or during surgical procedures.[1]