Is It True That Effexor (Venlafaxine) Acts As An SSRI At Low Doses?

Quick Answer

Effexor XR (venlafaxine extended-release) acts primarily as a selective serotonin reuptake inhibitor (SSRI) at lower doses of 75mg per day or less. At higher doses of 150mg per day or more, it functions more like a serotonin-norepinephrine reuptake inhibitor (SNRI), although some studies suggest that norepinephrine reuptake inhibition does not significantly start until 225mg per day.

Detailed Answer

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Effexor XR (venlafaxine) is a medication used to treat depression and anxiety disorders. It is also used 'off-label' to treat a variety of other conditions, such as diabetic neuropathy and fibromyalgia. It is considered a serotonin-norepinephrine reuptake inhibitor (SNRI), which means it works by blocking the reuptake of serotonin and norepinephrine, two neurotransmitters that are involved in regulating mood.

However, studies show that the degree to which it inhibits the reuptake of these neurotransmitters can vary depending on the dose administered.

What Do The Studies Say?

Research has indicated that Effexor (venlafaxine) exhibits a considerably greater preference for inhibiting the reuptake of serotonin compared to norepinephrine (it is reported to have a 30-fold higher affinity for the reuptake inhibition of serotonin compared to norepinephrine). This is thought to be the highest serotonin-to-norepinephrine affinity ratio among all SNRIs (serotonin-norepinephrine reuptake inhibitors), which also includes drugs like Cymbalta and Fetzima.

At lower doses of 75 mg per day or less, Effexor XR is essentially acting as a selective serotonin reuptake inhibitor (SSRI). This means that it primarily inhibits the reuptake of serotonin and has limited effects on the reuptake of norepinephrine. However, at higher doses, such as 150mg (or higher) per day, Effexor XR has been found to have significant effects on the norepinephrine transporter, and therefore acts more like an SNRI.

One article on the matter, published in 'The Journal of Clinical Psychiatry' stated the following:

...venlafaxine (and its metabolite O-desmethylvenlafaxine) is quite likely to inhibit 5-HT up-take, even at the low end of its concentration in CSF [cerebrospinal fluid] . As its concentration in CSF rises, presumably a function of higher doses, venlafaxine begins to reach concentrations capable of blocking NE reuptake. Thus, venlafaxine would not appear to be an SSRI at higher doses. Unfortunately, it is unclear at what doses venlafaxine becomes noradrenergic. Some data suggest that this might occur at 150 mg but more studies are required. It is generally believed that nor-adrenergic effects occur at doses of 150 mg or higher.

A different study, published in 'The International Journal of Neuropsychopharmacology' suggests venlafaxine doesn't begin to appreciably affect norepinephrine until doses as high as 225 mg/day and 375 mg/day were used.

This study compared the effects of a low dose (75 mg) and a high dose (up to 375 mg per week) of the medication. A total of 44 patients with major depression were involved in the study and were assessed bi-weekly for the first two weeks and weekly for the next two weeks.

It also assessed the effects of venlafaxine on both 5-HT and NE reuptake. The inhibition of 5-HT reuptake was estimated by measuring the depletion of whole-blood 5-HT, while the inhibition of NE reuptake was assessed by measuring the attenuation of systolic blood pressure elevations produced by intravenous injections of tyramine.

The study found that at the minimal effective dose for treating depression (75 mg/day), venlafaxine acted as a selective 5-HT reuptake inhibitor, while at higher doses (225 and 375 mg/day), it acted as a dual 5-HT and NE reuptake inhibitor.

It is noteworthy to mention that the investigation did not reveal statistically significant discrepancies in depression scores across the various dosages administered.

This observation may hold significant implications in terms of discerning appropriate dosage for treating depression, which may be optimized at lower doses, versus higher dosages for conditions that are responsive to norepinephrine reuptake inhibitors, such as neuropathic pain.

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