Types of Antidepressants: How SSRIs, SNRIs, NDRIs, Tricyclics, and Atypical Antidepressants Differ

What Antidepressant Classes Are

Antidepressant classes are broad groups of medications that are organized mainly by mechanism of action: in other words, by which neurotransmitter systems they affect and how they do it. That classification is useful because drugs in the same class often share some broad tendencies in side effects, tolerability, and clinical use, even though they are not interchangeable. An SSRI is not simply a brand name version of an SNRI, and an older tricyclic is not just a stronger antidepressant from the same family. They work differently enough that class still matters in everyday prescribing.

At the same time, class labels are only a map. They do not tell the whole story about any single drug. Lexapro (escitalopram) and Zoloft (sertraline) are both SSRIs, but they are not identical in clinical feel. Duloxetine and venlafaxine are both SNRIs, but they are not used in exactly the same way in practice. Wellbutrin (bupropion) is often discussed both as an NDRI and, more loosely, as an atypical antidepressant because its profile differs so much from serotonergic drugs. And “atypical antidepressants” is really a practical catch-all category rather than a single coherent pharmacologic family.

That is exactly why doctors think about antidepressants on two levels at once: class-level patterns and individual-drug differences. The class tells you the general direction. The specific drug tells you the finer trade-offs around activation, sleepiness, sexual effects, appetite, and tolerability.

SSRIs: The Most Common Starting Point

Selective serotonin reuptake inhibitors, or SSRIs, are still the best-known and most commonly used antidepressant class. They increase serotonergic signaling by blocking the reuptake of serotonin, and they are widely used as first-line treatment because of their safety, tolerability, and familiarity in routine practice. StatPearls describes SSRIs as the class most commonly prescribed for depression and notes that they are often used as first-line pharmacotherapy because of that balance of efficacy and tolerability.

Two of the most familiar examples are Lexapro (escitalopram) and Zoloft (sertraline). These drugs are often the starting point when clinicians want an antidepressant with a relatively well-understood side-effect profile, broad experience base, and lower toxicity than older classes such as tricyclics.

The trade-off is that SSRIs have a characteristic adverse-effect pattern. They are generally easier to tolerate than tricyclics, but they are also strongly associated with sexual side effects, including lower libido, reduced arousal, erectile difficulty, and delayed or absent orgasm. Recent reviews continue to emphasize that sexual dysfunction remains one of the most clinically important burdens of serotonergic antidepressants because it affects both quality of life and adherence.

SSRIs can also be activating in some people, especially early in treatment, though that varies by drug and by patient. Some patients instead experience fatigue or sleep disruption. This is one reason class-level knowledge is helpful but incomplete: even within SSRIs, the real-world experience can differ from person to person.

SNRIs: Similar Role, Different Emphasis

Serotonin-norepinephrine reuptake inhibitors, or SNRIs, affect both serotonin and norepinephrine pathways. In broad clinical terms, they occupy a similar treatment tier to SSRIs in depression, but they have a somewhat different pharmacologic emphasis. Two major examples are duloxetine and venlafaxine.

Like SSRIs, SNRIs are used for major depressive disorder and several other psychiatric conditions. But some references continue to note that they may be especially relevant when depression coexists with pain symptoms, which is one reason duloxetine in particular often appears in discussions that bridge psychiatry and general medicine. Their side-effect profile overlaps with SSRIs in important ways. Because they still have a strong serotonergic component, SNRIs can also be associated with sexual dysfunction and other serotonin-related adverse effects. At the same time, the norepinephrine component means that some patients experience them as more activating, though that is far from universal. Venlafaxine is also often discussed as less forgiving around discontinuation symptoms than some other antidepressants, a point highlighted in recent clinical-practice guidance.

So, the practical difference between SSRIs and SNRIs is not that one class is modern and the other is niche, but that they share some clinical territory, but SNRIs may be chosen when the clinician wants that dual serotonin-norepinephrine profile and is comfortable with the somewhat different side-effect and withdrawal trade-offs.

NDRIs: Where Wellbutrin Fits

NDRI stands for norepinephrine-dopamine reuptake inhibitor, and in routine practice this category is represented mainly by Wellbutrin (bupropion). Bupropion is pharmacologically distinct from SSRIs and SNRIs, and that difference shows up clearly in how patients and clinicians think about it.

The most important practical difference is side-effect profile. Compared with serotonergic antidepressants, bupropion is often discussed as having a lower burden of sexual side effects, and recent outpatient data continue to support that impression. A 2025 outpatient study found that bupropion had a significantly lower rate of treatment-emergent sexual dysfunction than SSRIs such as escitalopram and sertraline.

Bupropion also stands apart in discussions of weight. A 2025 review on antidepressants and weight gain described mirtazapine and bupropion as being on opposite sides of the weight-gain spectrum, with bupropion generally associated with a more favorable weight profile. That does not mean everyone loses weight on it, but it is one reason bupropion often comes up when sexual side effects or weight gain are major priorities in treatment selection. Clinically, bupropion can feel more activating than sedating, which can make it appealing in some patients and less appealing in others. That is exactly why class matters: an NDRI is not just another antidepressant with a different brand name. It offers a different balance of sexual tolerability, weight effects, and activation.

Tricyclic Antidepressants: Older, Still Useful, Often Harder To Tolerate

Tricyclic antidepressants, or TCAs, are older antidepressants with broader receptor activity and a more burdensome adverse-effect profile than most newer agents. Two classic examples are amitriptyline and nortriptyline. They remain clinically important, but they are generally not the easiest antidepressants to live with.

One reason is tolerability. TCAs are well known for anticholinergic effects, sedation, and broader off-target receptor activity. That can make them useful in selected clinical situations, but it also makes them harder to tolerate than SSRIs for many patients. Large evidence summaries from AHRQ and NCBI have found that SSRIs lead to fewer withdrawals from adverse events than TCAs in acute-phase treatment of major depression, which fits long-standing clinical experience. Safety also matters. StatPearls notes that older tricyclic antidepressants are more toxic than newer antidepressant classes such as SSRIs, and that toxicity in overdose is generally higher with TCAs. That issue alone affects prescribing decisions, especially when suicide risk is part of the clinical picture.

None of this makes TCAs obsolete. It makes them more selective tools. In practice, clinicians may still choose amitriptyline or nortriptyline when the symptom pattern, prior history, or clinical context makes them appropriate. But compared with SSRIs, they usually come with more sedation, more anticholinergic burden, and less forgiving safety.

Atypical Antidepressants: A Mixed Group With Distinct Uses

“Atypical antidepressants” is less a single pharmacologic class than a practical bucket for drugs that do not fit neatly into the SSRI, SNRI, or TCA categories. The most relevant examples here are mirtazapine and trazodone, though broader reference sources also often place bupropion in this atypical group.

What makes this category tricky is that the drugs in it differ sharply from one another. Mirtazapine is commonly associated with sleepiness, increased appetite, and weight gain. Recent real-world safety research from 2026 still lists somnolence, weight gain, dizziness, and increased appetite among the adverse events most commonly associated with mirtazapine, which fits its established clinical profile.

Trazodone has a different place in practice. It is often viewed as a more sedating antidepressant and is frequently discussed in relation to sleep, even though it is also an antidepressant in its own right. Older clinical literature and review summaries continue to note that trazodone is often used in combinations or in contexts where sedation is relevant, which is very different from how people usually think about activating drugs like bupropion.

The main point is that atypicals are chosen when clinicians want a different trade-off. Mirtazapine may be attractive when insomnia or poor appetite are prominent. Trazodone may be considered when sedation is part of the intended balance. This category is therefore less unified by mechanism than by clinical usefulness in specific symptom patterns.

How Side Effects Differ Across Classes

Across classes, some side-effect patterns stand out clearly enough to guide real prescribing. Sexual side effects are most often associated with the more serotonergic classes, especially SSRIs and also SNRIs. Recent reviews and outpatient data continue to support that pattern, while bupropion remains one of the better-known lower-risk options in this domain.

For weight, the broad picture is that mirtazapine and many tricyclics tend to raise more concern about gain, while bupropion tends to have a more favorable profile. A 2025 review explicitly highlighted mirtazapine and bupropion as being on opposite ends of the spectrum.

On the question of sleepiness versus activation, mirtazapine, trazodone, and many TCAs are generally more associated with sedation, while bupropion is more often viewed as activating. SSRIs and SNRIs sit in the middle: some patients feel more alert or restless on them, while others feel tired or dulled.

For overall tolerability, SSRIs still tend to be easier than TCAs, especially when safety and overdose risk are considered.

Why One Class May Be Chosen Over Another

Doctors do not choose an antidepressant class by mechanism alone. They choose based on the fit between the drug’s profile and the patient’s symptoms, treatment history, side-effect priorities, comorbidities, and tolerance for specific trade-offs. Recent guideline language emphasizes patient-centered care and the need to account for individual needs and preferences rather than treating pharmacotherapy as one-size-fits-all. That is why a clinician may lean toward an SSRI as a familiar first-line option, toward an SNRI when pain symptoms are part of the picture, toward bupropion when sexual side effects or weight gain are major concerns, or toward mirtazapine when insomnia and poor appetite are part of the depressive syndrome. Older drugs like amitriptyline or nortriptyline may still be considered when their specific strengths outweigh their tolerability burden.

In that sense, class differences are not abstract pharmacology. They are one of the main ways doctors translate a patient’s symptom pattern into a practical treatment choice.

What This Means

Antidepressant classes are useful because they capture real differences in mechanism, side effects, and clinical fit. SSRIs such as Lexapro and Zoloft are often the default starting point because they are familiar and broadly tolerated. SNRIs such as duloxetine and venlafaxine add a norepinephrine component and may be especially relevant in some symptom patterns, including depression with pain. Wellbutrin (bupropion) stands out for its different profile around sexual side effects, weight, and activation. Amitriptyline and nortriptyline remain important but are generally harder to tolerate and riskier in overdose. Mirtazapine and trazodone are useful precisely because they offer different balances around sleep, appetite, and sedation. The final choice still depends on the person: symptoms, history, side-effect priorities, and prior response matter as much as class itself.

References

1. Mouawad, M., Hallit, S., Haddad, C., Hallit, R., Karam, C., Hallit, J., Zein, M., Salameh, P., & Sacre, H. (2025). Impact of antidepressants on weight gain. Fortune Journals. https://pmc.ncbi.nlm.nih.gov/articles/PMC12121960/
2. Safak, Y., Kucukkarapinar, M., Demirkol, M. E., & Kus, B. (2025). Antidepressant-associated sexual dysfunction in outpatients. International Journal of Impotence Research. https://pmc.ncbi.nlm.nih.gov/articles/PMC11967063/
3. Sheffler, Z. M., Patel, P., & Abdijadid, S. (2023). Antidepressants. In StatPearls. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK538182/
4. U.S. Department of Veterans Affairs & U.S. Department of Defense. (2022). VA/DoD clinical practice guideline for the management of major depressive disorder. https://www.healthquality.va.gov/guidelines/MH/mdd/VADODMDDCPGFinal508.pdf
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