Effexor is a type of antidepressant known as a selective norepinephrine reuptake inhibitor (SNRI). This type of medication blocks the reabsorption of serotonin and norepinephrine in the brain and overdoses can lead to a disorder known as serotonin syndrome. This syndrome is characterized by altered mental status and can be a life-threatening disorder when left untreated. Antidepressants can be effective for some canines in situations such as severe anxiety or cognitive dysfunction, but even therapeutic doses can result in moderate signs of toxicity.

Effexor is an antidepressant that blocks the reabsorption of serotonin and norepinephrine in the brain. Overdoses can lead to a disorder known as serotonin syndrome.

Signs and symptoms in dogs that have taken the extended release version of Effexor (Effexor XR) may have a delayed onset and may persist for up to 72 hours.

Pain medications taken in conjunction with an SNRI like Effexor can cause your pet to bleed or bruise easily.

SNRI medications may occasionally be prescribed for a canine with severe anxiety or compulsive type disorders, although it can be 1-4 weeks before full effects are seen. 

SNRI medications and their brand names:

Other types of antidepressants that can cause serotonin syndrome: 

SNRI medications like Effexor are designed to change behavior by blocking the reabsorption of serotonin and norepinephrine in the brain. In some cases, this can cause an overabundance of serotonin in the brain and the resultant overstimulation of the receptors in the brain can cause a cascade of symptoms related to the disruption of the central nervous system (CNS). This is known as serotonin syndrome, and can occur either from an overdose of an antidepressant medication or a combined effect of prescribed anti-depressants and foods or supplements that can increase the levels of serotonin in the brain.

SAMe (S-adenosylmethionine), a supplement commonly used for mental, liver, and joint support, and is known to increase the levels of serotonin and should not be given in conjunction with an SNRI or SSRI antidepressant.  5HTP (5-hydroxytryptophan) is an amino acid that is used by the body to manufacture serotonin, and supplementation with 5HTP while taking antidepressants can cause a toxic increase in serotonin levels. Foods high in tryptophan and Vitamin B6, B9, and D are also known to stimulate serotonin production, as is magnesium.

Your veterinarian will need to get your pet’s full history from you, including any prescriptions or supplements that you give your dog. If you witnessed the ingestion of the antidepressant you should give your veterinarian as much information about amounts and dosages that you can, and bring in any packaging from the medication as well. A chemistry profile and complete blood count will be completed as well during the general physical examination. Although there is no direct test for serotonin syndrome itself, the urine, stools, and vomitus testing may expose underlying diseases or toxins as well as the venlafaxine itself.  If the stomach of the canine is pumped during the visit, any gastric contents will also be thoroughly analyzed. Neurological testing may also occur during the diagnostic appointment, in order to measure your pet’s reflexes and coordination. This may also be helpful in pinpointing the specific areas of the nervous system that have been affected.

Always contact your veterinarian immediately if you catch your pet consuming anything that they are not supposed to. In the case of Effexor and other antidepressants your veterinarian may instruct you on the proper way to cause your canine to vomit before traveling to the office if it was ingested within the last thirty minutes, to avoid any further absorption of toxic amounts into the bloodstream. If it has been longer than thirty minutes, gastric irrigation under general anesthesia may be performed at the veterinary office. This is done to remove as much of the medication from the digestive system as possible before it can be absorbed into the blood. Fluids and supportive treatment will also be given, including regulation of the body temperature, IV fluids for dehydration, and serotonin antagonists.  Prognosis of serotonin syndrome is dependent on the amount eaten, the speed of initial diagnosis and treatment, and the size of the dog, but in most cases that are treated promptly, recovery is seen within twelve to eighteen hours from ingestion. If the medication taken was the extended release formula, recovery is still likely but it may take 48 to 72 hours to clear the system.

Patients that are in recovery from requiring anesthesia for a gastric lavage may have coordination difficulties when they first get home and are often quite disoriented. Ensuring that the recuperating patient has a calm and quiet environment with ample access to highly nutritional food and clean water to return home to will help speed recovery, and isolation from children and other pets is generally recommended until the anesthesia and the Effexor has fully cleared your companion’s system. It is important to ensure that your pet completes the full measure of any medications that were prescribed for management of the condition, and any other supplements should be discussed with your veterinarian to avoid negative interactions.