Toxicology
Talk
Column | Peer Reviewed

Trazodone in Veterinary Medicine

Toxicology Talk is written and reviewed by members of the American Society for the Prevention of Cruelty to Animals (ASPCA) Animal Poison Control Center (APCC). The mission of the APCC is to help animals exposed to potentially hazardous substances, which it does by providing 24-hour veterinary and diagnostic treatment recommendations from specially trained veterinary toxicologists. It also protects and improves animal lives by providing clinical toxicology training to veterinary toxicology residents, consulting services, and case data review.

The ASPCA APCC includes a full staff of veterinarians, including board-certified toxicologists, certified veterinary technicians, and veterinary assistants, and its state-of-the-art emergency call center routinely fields requests for help from all over the world, including South America, Europe, Asia, and the Pacific Islands.

Background

Trazodone is a serotonin 2A antagonist and reuptake inhibitor that has been used in human medicine as a prescription therapy for depression, aggression, sleeplessness, and anxiety since 1981.1–3 It is available in 50-, 100-, 150-, and 300-mg tablets as well as 150- and 300-mg extended-release tablets.1 No products are labeled for veterinary use.

Trazodone selectively blocks serotonin reuptake, which enhances serotonin’s effects.4 It is an antagonist of 5-HT2A, H1-histaminic, and α1-adrenergic receptors at low to moderate doses, resulting in various levels of sedation.3–6 It can have hypotensive effects.4 At higher doses, trazodone acts as a serotonin agonist, and serotonin syndrome can develop.4,6 Trazodone also has anxiolytic properties, but the exact mechanism of action is unknown.4

Use in Veterinary Medicine

In 2008, Gruen and Sherman studied 56 dogs prescribed trazodone in combination with other primary behavior therapies and discovered that trazodone seemed to offer therapeutic benefit with relatively minimal adverse effects.7 Since then, studies have investigated the benefit of trazodone in postorthopedic surgery treatment plans involving confinement to enhance calm behavior and reduce anxiety in hospitalized dogs. Trazodone has generally been shown to be beneficial and relatively safe. Adverse events associated with trazodone can be divided into behavioral and systemic signs. Adverse events previously reported in the literature include drugged or “spacy” behavior, drowsiness, panting, anxiety/restlessness/agitation, vomiting/gagging, behavioral change (counter surfing and trash raiding), excitation, sedation, increased hunger, colitis, and aggression (growling).5,7,8

In veterinary medicine, trazodone is generally dosed at 1.7 to 19.5 mg/kg/d on a daily or as-needed basis with immediate action (not extended-release) tablets and can be given with food.1 When administered in combination with tricyclic antidepressants or selective serotonin reuptake inhibitors, it is recommended to begin dosing trazodone at 2 to 5 mg/kg and increase as needed to a maximum dose of 14 mg/kg/d.1 Trazodone should be administered about an hour before potential anxiety-inducing stimuli, as its onset of action is approximately 30 to 60 minutes.1,5 Gruen and colleagues reported owner-observed duration of effect lasting 4 hours or more.5 The parent compound has an elimination half-life of approximately 7 hours in immediate-release tablets.1 Trazodone undergoes extensive metabolism in the liver and is predominately excreted via the kidneys.1,4

Exposure

Incidence and Clinical Signs

The ASPCA Animal Poison Control Center reported 417 incidences involving single-agent trazodone exposures in 379 dogs from 2009 to 2013.1 In 104 dogs experiencing adverse effects, sedation and lethargy were reported in 43% of the dogs. Ataxia was reported in 16% and vomiting in 14%. Overall, lethargy, sedation, depression, somnolence, and subdued behavior are considered common signs of trazodone exposure.9 Additional information on signs reportedly exhibited by dogs exposed to trazodone alone from January 2003 to November 2016 and the lowest dose at which each sign was seen is provided in TABLE 1.

TABLE 1 Signs Associated With Trazodone Exposure in Dogs10

SIGN Lowest Dose (mg/kg) at Which Sign Was Seen
Lethargy 0.55
Depression/vomiting 1.35
Ataxia 1.7
Diarrhea 2.82
Hyperactivity 3.8
Hypotension 5.94
Hyperesthesia 6.06
Vocalization 6.6
Tremors 8.17
Disorientation 8.28
Tachycardia/hypertension 8.83
Hyperthermia 11.8
Collapse 12.99
Mydriasis/bradycardia 16.23
Seizure 78.7

Management

Decontamination measures are an important component of exposure management. Induction of emesis within 1 hour of exposure is recommended in asymptomatic patients if no contraindications to emesis exist.9 Activated charcoal with sorbitol may be recommended in large exposures only.9

Treatment of trazodone overdose generally consists of symptomatic and supportive care. Although adverse effects are often reported, relatively few cases have involved serious signs and no deaths are attributed to trazodone exposure to date.9 Special attention should be given to ensuring maintenance of cardiac output and being attentive to signs of hyperthermia or hypothermia and correcting as needed.9,11 IV fluid therapy may be needed to maintain blood pressure.6,9 Diazepam is the drug of choice for managing tremors or seizures, and atropine is suggested for treatment of bradycardia.6,9

Being aware of signs associated with serotonin syndrome is important as this can be a serious and potentially life-threatening condition to manage. Serotonin syndrome develops as a result of an overabundance of serotonin in the central nervous system.6 It can be a risk in cases of exposure to high doses of trazodone.1,4 Clinical signs of serotonin syndrome in dogs, in order of most to least common, include vomiting, diarrhea, seizures, hyperthermia, hyperesthesia, depression, mydriasis, vocalization, death, blindness, hypersalivation, dyspnea, ataxia/paresis, disorientation, hyperreflexia, and coma.1 Cyproheptadine, a serotonin antagonist, helps combat serotonin syndrome signs.6,11 Phenothiazines should be used cautiously because of potential hypotensive effects.6

Interaction With Other Drugs

Trazodone interacts with numerous drugs, and some of these interactions may have clinically significant effects. Of special interest are medications that may be strong inhibitors or inducers of cytochrome P450 3A4 (CYP3A4) isoenzyme, which is involved in trazodone metabolism.4 Azole antifungals (eg, ketoconazole, itraconazole, fluconazole) and macrolide antibiotics (eg, erythromycin, telithromycin, clarithromycin) are CYP3A4 inhibitors and thus may enhance the effect of trazodone.1,4 Carbamazepine, phenobarbital, phenytoin, rifampicin, and modafinil, which are CYP3A4 inducers, may decrease the effect of trazodone.1,4,9

Extreme caution should be used with concomitant trazodone and fluoxetine use and/or exposure as it is believed that fluoxetine may inhibit metabolism of trazodone.1,4 Additionally, fluoxetine and other serotonergic medications (eg, paroxetine, sertraline, amitriptyline, clomipramine, amphetamines, dextromethorphan) as well as monoamine oxidase inhibitors (eg, phenelzine, amitraz, selegiline), metoclopramide, and tramadol, could heighten the risk of serotonin syndrome when combined with trazodone.1,4,6,8

Serotonin syndrome patients with severe cardiac disease or renal and/or hepatic function deficits should be monitored very closely and may merit additional treatment measures.1

Prognosis

The prognosis for patients exposed to trazodone is generally good, especially when serotonin syndrome has not developed. Signs generally resolve in 12 to 24 hours.6

Show MoreReferences
  1. Plumb DC. Trazodone HCl. In: Veterinary Drug Handbook. 8th ed. Ames: John Wiley and Sons; 2016.
  2. Trazodone (Poisindex Managements). In: POISINDEX® System (electronic version). Greenwood Village, CO: Truven Health Analytics. Available at: micromedexsolutions.com. Accessed December 2016.
  3. Gruen ME. Beyond the front line: trazodone and other ancillary treatments for anxiety. Vet Med Forum May 2012 cvm.ncsu.edu/wp-content/uploads/2015/06/Gruen_VetMedForum_May2012.pdf. Accessed December 2016.
  4. McEvoy GK, ed. Trazodone. In: AHFS Drug Information 2014. Bethesda, MD: American Society of Health-System Pharmacists; 2014:2450-2454.
  5. Gruen ME, Roe SC, Griffith E, et al. Use of trazodone to facilitate postsurgical confinement in dogs. JAVMA 2014;245:296-301.
  6. Wismer TA. Antidepressant drug overdoses in dogs. Vet Med 2000;95(7):520-525.
  7. Gruen ME, Sherman BL. Use of trazodone as an adjunctive agent in the treatment of canine anxiety disorders: 56 cases (1995-2007). JAVMA 2008;233:1902-1907.
  8. Gilbert-Gregory SE, Stull JW, Rice MR, et al. Effects of trazodone on behavioral signs of stress in hospitalized dogs. JAVMA 2016;249:1281-1291.
  9. ASPCA Animal Poison Control Center. Trazodone vet protocol. Unpublished data.
  10. ASPCA Animal Poison Control Center. AnTox database. Unpublished data.
  11. Wismer TA. Antidepressant drug overdoses. Vet Tech 2006;27(5):278-281.
Close