Atypical antidepressants explained

What atypical antidepressants are

"Atypical antidepressants" is not a tidy category. It is a loose, catch-all label for antidepressants that do not fit the two big classes, the SSRIs and the SNRIs. The medications grouped under it work in different ways from one another. The term describes what they are not, more than what they share.

That is the most important thing to understand about the group. There is no single atypical mechanism, no shared side-effect profile, no common feel. Bupropion and mirtazapine, for instance, are almost opposites: one is activating and weight-neutral, the other is sedating and increases appetite. The useful way to approach this class is one medication at a time.

How they work

Each atypical antidepressant works through its own mechanism, and the niche each one fills follows from that mechanism.

Bupropion is a norepinephrine-dopamine reuptake inhibitor, usually shortened to NDRI. It slows the reuptake of norepinephrine and dopamine, two chemical messengers, so more of each stays available between nerve cells. It does not act much on serotonin at all. That is why it tends to be activating and why it rarely causes the sexual side effects that come with serotonin-based antidepressants.

Mirtazapine works differently again. Rather than slowing reuptake, it blocks certain receptors, the docking sites on cells, in a way that ends up increasing the activity of norepinephrine and serotonin. It also strongly blocks histamine, which is why it is sedating and why it increases appetite.

Trazodone affects serotonin and blocks certain receptors, and that receptor blocking is what makes it sedating. The dose changes what it does. Low doses are used for the sedating effect, higher doses for the antidepressant effect.

Vortioxetine is a newer multimodal agent, meaning it works through more than one mechanism. It slows serotonin reuptake like an SSRI, but it also acts directly on several serotonin receptors, turning some up and some down. That broader action is why it is grouped here rather than with the plain SSRIs.

As with all antidepressants, the full picture of how these medications ease depression is not known, and the early changes in signaling are thought to set off slower adjustments in the brain over the following weeks.

How the class developed

"Atypical antidepressant" is a label history left behind, not a class anyone set out to design. The members were developed at different times and by different routes, and they were grouped together mainly because they did not fit the SSRI or SNRI categories that organized everything else.

Bupropion and trazodone both arrived in the 1980s, around the same era as the early SSRIs, each from its own line of research. Mirtazapine followed in the 1990s. Vortioxetine is much newer, one of a group of multimodal agents that reached the market in the 2010s. So the group spans more than three decades of drug development. What its members have in common is mostly that they sit outside the two main classes.

What they treat

Every medication in this group is, at its core, an antidepressant approved for major depressive disorder. Beyond that, the uses fan out.

Bupropion is also approved to prevent seasonal affective disorder, a depression that follows the seasons, and, under the brand name Zyban, to help people stop smoking. Mirtazapine is often used off-label for anxiety and for insomnia, meaning for purposes the label doesn't formally list even though evidence and practice support them.

Trazodone deserves a clear statement. Trazodone is FDA-approved as an antidepressant. In everyday practice, though, it is most often prescribed at low doses, off-label, as a sleep aid. The antidepressant role is the official one. The off-label use is the low-dose sleep use. The dose tells you which job it is doing.

Vortioxetine is approved for major depressive disorder.

What they have in common

The medications in this group differ a great deal, but they still share a few things. They usually take four to six weeks for the fuller effect on mood, as other antidepressants do. They carry the same antidepressant boxed warning about a possible increase in suicidal thoughts in people under 25, especially early in treatment. And none of them are addictive in the usual sense. They do not cause cravings or compulsive use, though the body does adjust to them, so stopping is generally done gradually with a prescriber.

Beyond that, the group has little in common, which is the whole point of the label.

How they differ from each other

The differences here are larger than within any other antidepressant class, and they are the reason a prescriber reaches for one of these medications in particular.

Bupropion is activating. It rarely causes sexual side effects, and it tends not to cause weight gain. It can even reduce appetite. Against that, it can worsen anxiety or insomnia, and it carries a dose-related seizure risk, so it is not used in people with a seizure disorder or a current or past eating disorder.

Mirtazapine is sedating and tends to increase appetite. Those features can be a drawback, or a deliberate benefit, depending on the person. It also has a low rate of sexual side effects and causes less nausea than SSRIs.

Trazodone is sedating, weight-neutral, and not a controlled substance. It does not carry the dependence risk of benzodiazepines or z-drugs, which is part of why it is so widely used for sleep.

Vortioxetine is a newer multimodal agent with a lower rate of sexual side effects than many SSRIs and SNRIs. Nausea is its most common side effect, and it is roughly weight-neutral.

How a prescriber chooses one

A prescriber usually reaches for an atypical antidepressant when an SSRI or SNRI is not a good fit, or when one of those medications helped only partly. From there, the choice tends to follow a specific feature.

If sexual side effects on an SSRI have been a problem, bupropion or vortioxetine becomes attractive, since both have a lower rate. Sometimes bupropion is even added alongside an SSRI specifically to counter that side effect. If depression brings severe insomnia and weight loss, mirtazapine fits the picture, because its sedation and appetite increase work in the person's favor. If sleep is the main problem, low-dose trazodone is a common choice that avoids the dependence risk of standard sleeping pills. If low energy and motivation dominate, the activating profile of bupropion can suit.

The point of the group is choice. Having medications that work differently from the standard options gives a prescriber a way to match the medication to the person when a first-line drug has not done the job.

The medications in this class

• Bupropion (Wellbutrin). A norepinephrine-dopamine reuptake inhibitor. Activating, rarely causes sexual side effects, tends not to cause weight gain, and carries a dose-related seizure risk.
• Mirtazapine (Remeron). Sedating and appetite-increasing, often chosen when depression comes with severe insomnia and weight loss.
• Trazodone (Desyrel). An FDA-approved antidepressant most often prescribed at low, off-label doses as a sleep aid.
• Vortioxetine (Trintellix). A newer multimodal antidepressant with a lower rate of sexual side effects than many SSRIs.
• Vilazodone (Viibryd). A newer antidepressant that acts on serotonin in a more complex way than a standard SSRI.

PsychiatryRx has a dedicated guide for several of these medications, with more detail on uses, side effects, dosing, and what to expect.

Common questions

What does "atypical" actually mean? It means the medication does not fit the SSRI or SNRI categories. It is not a description of how a drug works, because the atypical antidepressants do not share a mechanism. It is a leftover label for everything outside the two main classes. The useful way to understand these medications is one at a time.

Why is bupropion so different from the others? Bupropion does not act much on serotonin. It works on norepinephrine and dopamine instead. That single fact explains most of its profile: it tends to be activating rather than calming, it rarely causes the sexual side effects tied to serotonin medications, and it does not tend to cause weight gain. Its main caution, a dose-related seizure risk, also follows from how it works.

Why is trazodone used for sleep? Trazodone is sedating because of the way it blocks certain receptors. At low doses, that sedating effect is useful for sleep, and it works the same night rather than taking weeks. Trazodone is an FDA-approved antidepressant, but the antidepressant effect needs much higher doses. So the low-dose sleep use is off-label, even though it is the most common reason trazodone is prescribed.

Sources

This guide draws on current prescribing information and public health references. It is reviewed for clinical accuracy and updated as guidance changes.

1. U.S. Food and Drug Administration. Prescribing information.
2. MedlinePlus, U.S. National Library of Medicine.
3. National Institute of Mental Health. Mental health medications.
4. American Psychiatric Association. Practice guideline for the treatment of patients with major depressive disorder.