Trazodone is one of the most commonly prescribed anti-anxiety medications for dogs, and about 80% of dogs tolerate it without any noticeable side effects. For the remaining 20%, the most frequent issues are sedation, lethargy, nausea, increased appetite, and diarrhea. Most of these are mild and resolve on their own, but there are a few rarer reactions worth knowing about.
The Most Common Side Effects
In a review of 104 dogs that experienced adverse effects from trazodone, sedation and lethargy were by far the most reported, showing up in 43% of affected dogs. Unsteadiness or wobbliness when walking (ataxia) appeared in 16%, and vomiting in 14%. Your dog may also seem “spacy” or unusually drowsy, which is essentially the medication doing its job a bit too aggressively.
Other commonly reported effects include panting, excessive thirst, soft stools or diarrhea, constipation, and a noticeable spike in hunger. Some dogs become surprisingly food-motivated, raiding trash cans or counter-surfing in ways they normally wouldn’t. If your dog seems nauseous, giving trazodone with food often helps reduce that.
Paradoxical Reactions: Agitation and Aggression
Some dogs react to trazodone in the opposite way you’d expect. Instead of calming down, they become more anxious, restless, agitated, or even aggressive. This is called a paradoxical reaction, and while it’s uncommon, it’s well-documented.
In one study of hospitalized dogs, a dog with no prior history of aggression began growling, lunging, and snapping about 60 minutes after receiving trazodone. The behavior stopped after the medication wore off, but when the dog was given a second dose the following day, the exact same aggressive response appeared at the same time point. Aggression has been noted as a side effect in multiple studies, though it remains rare. If your dog becomes agitated or aggressive after a dose, that’s a sign trazodone isn’t the right fit.
Serotonin Syndrome: The Serious Risk
The most dangerous potential side effect of trazodone is serotonin syndrome, a condition where serotonin levels in the brain climb dangerously high. This is rare with trazodone alone but becomes a real concern when it’s combined with other medications that also raise serotonin levels.
Signs of serotonin syndrome in dogs, from most to least common, include:
- Vomiting and diarrhea
- Seizures
- High body temperature
- Extreme sensitivity to touch
- Dilated pupils
- Vocalization
- Difficulty breathing
- Disorientation or coma
Serotonin syndrome can be fatal. There is no specific antidote for trazodone, so treatment is supportive, meaning the focus is on managing symptoms until the drug clears your dog’s system. If you notice any combination of these signs, especially seizures, a high temperature, or difficulty breathing, it’s an emergency.
Drug Interactions That Increase Risk
The risk of serotonin syndrome jumps significantly when trazodone is given alongside other medications that affect serotonin. These include common veterinary prescriptions like fluoxetine (Prozac for dogs), other antidepressants in the SSRI or tricyclic categories, and certain pain medications like tramadol. Dogs should also not take trazodone with MAO inhibitors, which includes some flea and tick products containing amitraz.
Trazodone can also lower blood pressure. If your dog is already on a medication that reduces blood pressure, combining it with trazodone can cause an unsafe drop. Make sure your vet knows every medication and supplement your dog is taking before starting trazodone.
How Side Effects Relate to Dose
Side effects tend to track with how much trazodone a dog receives relative to their body weight. Data from adverse exposure reports shows a pattern: lethargy can appear at very low doses (around 0.5 mg/kg), vomiting and depression at roughly 1.35 mg/kg, wobbliness at about 1.7 mg/kg, and diarrhea at around 2.8 mg/kg. The standard therapeutic range for dogs is 4 to 18 mg/kg per dose, with a maximum of 300 mg, so some degree of sedation is expected and often intentional.
Dogs under 40 pounds are typically started at the lower end of this range, while larger dogs often begin at a flat 100 mg dose that gets adjusted upward if needed. If your dog seems overly sedated or is having trouble walking, the dose may simply be too high for them. This is especially true the first time they take it, before you and your vet know how your individual dog responds.
What to Watch For After a Dose
Most side effects appear within the first one to two hours after your dog takes trazodone. Mild drowsiness and a calm, slightly “out of it” demeanor are usually the goal of the medication, not a cause for concern. The line between a therapeutic effect and a side effect is really about degree: a relaxed dog is the intended outcome, while a dog that can’t stand up or won’t respond to you has tipped into excessive sedation.
Keep an eye out for vomiting, especially in the first hour, and for any sudden behavioral shifts like agitation or aggression. These paradoxical reactions tend to show up quickly and consistently, so if it happens once, it will likely happen again with the next dose. Contact your vet before giving another dose if you see anything that concerns you, particularly signs that could point to serotonin syndrome like trembling, a rapid heart rate, or an unusually high body temperature.