Selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs) are two different types of antidepressants. SSRIs increase serotonin levels in the brain, while SNRIs increase both serotonin and norepinephrine levels.

Both SSRIs and SNRIs act on neurotransmitters, which are chemicals in the brain that help carry nerve signals between neurons.

Keep reading to learn more about the differences between these two types of medication, including how they work, the conditions that doctors may use them to treat, and the associated side effects.

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Cells in the brain make neurotransmitters, including serotonin and norepinephrine, which some people may call noradrenaline. Neurons, which are cells in the brain, send messages to one another using neurotransmitters. Neurotransmitters carry nerve signals across a cell synapse.

Different neurotransmitters carry different types of signals. Some, called excitatory neurotransmitters, increase the chance of the neuron generating an electrical signal called an action potential. Inhibitory neurotransmitters, by contrast, weaken or slow nerve signals.

Serotonin can serve both excitatory and inhibitory roles, while norepinephrine is a purely excitatory neurotransmitter.

SNRIs and SSRIs prevent the reuptake of certain neurotransmitters in the brain’s nerve terminals. SSRIs block serotonin reuptake, while SNRIs stop the reuptake of both serotonin and norepinephrine.

The reuptake process reduces the availability of neurotransmitters because the brain reabsorbs them. Therefore, by blocking this process, reuptake inhibitors increase the availability of neurotransmitters.

Serotonin helps the body regulate mood. When serotonin is too high or too low, a person may become depressed. Low serotonin can also cause anxiety, suicidal thoughts, depression, or obsessive-compulsive behaviors. When serotonin levels rise, a person may feel less physically and psychologically active or aroused, making it easier to sleep and relax.

SSRIs and SNRIs both increase serotonin levels in the brain, potentially helping with the symptoms of various mental health conditions.

Norepinephrine plays a key role in the fight-or-flight reaction. During times of intense stress, such as when a person feels threatened, norepinephrine naturally increases.

In daily life, norepinephrine also plays a role in sleep, attention, memory, and mood regulation. People with low levels of norepinephrine may have trouble concentrating, develop symptoms of attention deficit hyperactivity disorder (ADHD), or show signs of depression.

SNRIs increase norepinephrine levels, helping a person concentrate and reducing depression.

Higher levels of norepinephrine may cause feelings of euphoria. However, they may also cause panic attacks, high blood pressure, and hyperactive behavior. Due to these effects, SNRIs may not be suitable for people with certain heart health issues, a history of panic, or problems with hyperactivity.

There is no conclusive evidence that either group of drugs is consistently superior to the other for all individuals. A person may need to experiment with different drugs or combinations of drugs at different dosages.

Both classes of drugs can take several weeks to work.

Both SNRIs and SSRIs play a wide variety of roles in a person’s health and daily functioning. For example, serotonin helps regulate a person’s waking and sleeping rhythms, appetite, digestion, and endocrine system.

These diverse functions mean that SSRIs and SNRIs can cause a wide range of side effects.

The specific side effects that people experience vary among individuals, and they may also change over time or with higher dosages. Some people find certain side effects more troubling than others, so it is important to make a doctor aware of any specific concerns.

In some cases, a person may experience no side effects, or the side effects may go away as their body adjusts to the drug.

Some of the most common side effects of SSRIs include:

  • sexual dysfunction, including trouble becoming aroused, changes in libido, and difficulty reaching orgasm
  • unusual dreams
  • changes in sleep
  • fatigue
  • sinus infections and a sore throat
  • sweating
  • anxiety
  • hot flashes
  • rash

In children and adolescents:

  • muscle tremors or shaking
  • thirst
  • nose bleeds
  • heavy menstrual periods
  • increased urination

Although less common, some more serious side effects can also occur. These include:

  • suicidal thoughts or worsening depression, particularly among children and teenagers
  • allergic reactions
  • eye pain and changes in sight
  • rash
  • manic episodes
  • seizures
  • changes in weight or appetite
  • an irregular heart rate
  • serotonin syndrome, which is a potentially life threatening condition

Serotonin syndrome causes dizziness, a rapid heart rate, changes in blood pressure, and other symptoms in people with too much exposure to serotonin. It may be more likely to affect those taking multiple drugs that affect serotonin levels.

SNRIs can cause all of the same side effects as SSRIs.

They may also cause:

Some people may also experience symptoms of withdrawal after they stop taking either class of drugs.

Learn more about how to cope with antidepressant withdrawal here.

Some of the conditions that SNRIs and SSRIs can treat include:

Some SNRIs can treat diabetic neuropathy. Doctors may also use these drugs on an off-label basis to treat other conditions, such as sleep disorders.

While many drugs sit within one of these two categories, each specific drug behaves a little differently in the body. A person who does not like the side effects of one member of a drug group might still do well on a different drug that belongs to the same category.

A 2014 analysis of SNRIs, for example, found significant differences between the drugs in terms of side effects, the specific conditions for which the Food and Drug Administration (FDA) approved the drugs, and their effects on other neurotransmitters, such as dopamine.

Some examples of SNRIs include:

  • venlafaxine (Effexor XR)
  • duloxetine (Cymbalta)
  • desvenlafaxine (Pristiq)
  • milnacipran (Savella)
  • levomilnacipran (Fetzima)

Some SSRIs include:

Both SNRIs and SSRIs can help treat a range of mood issues, as well as some other conditions, such as chronic pain.

When a person takes these drugs under the supervision of a doctor, they can dramatically improve the quality of life.

However, too much of any neurotransmitter can negatively affect health and well-being. Therefore, people seeking mental health treatment should share with their doctor all of their symptoms, their complete medical history, and a list of all the drugs that they are taking, including supplements and over-the-counter medications.

It may take time and a little experimentation to find a medication that works, so people should try to work with a doctor who is willing to explore all options and listen carefully to concerns about side effects.