SSRIs and SNRIs are the most common classes of antidepressant. Despite some similarities, there are important differences, too.

If you’ve been reading about depression, chances are you’ve come across two essential drugs: selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs).

Antidepressants are drugs prescribed to help treat depression and other mental health conditions. They affect how the brain’s chemicals — specifically neurotransmitters — interact in your brain.

Neurotransmitters are the chemical messengers in the brain that can send information from one nerve cell (neuron) to another. Your brain depends on these neurotransmitters to function and regulate your moods.

And while the cause of depression is unknown, one of the contributing factors is believed to be low levels of certain neurotransmitters (such as serotonin, dopamine, and norepinephrine) in the brain. Antidepressants aim to change how your neurotransmitters transmit signals in the brain.

The brain sends signals using chemical messengers called neurotransmitters. A neuron releases neurotransmitters, which travel to another neuron, which absorbs the neurotransmitters in a process called reuptake.

Both SSRIs and SNRIs are types of antidepressants that slow the reuptake of certain neurotransmitters. This, in turn, affects the levels of those neurotransmitters in your brain. But while they work similarly in the brain, they target different neurotransmitters.

SSRIs work by boosting serotonin levels in the brain. They do this by slowing reuptake. By absorbing less serotonin, a higher level is available in your brain that your body can use for other purposes. This boosted level of serotonin is believed to lessen some symptoms of depression.

Some of the common types of SSRIs include:

SNRIs are a newer class of antidepressant drugs that affect two different neurotransmitters instead of only one: serotonin and norepinephrine.

There are seven SNRIs currently available:

Both SSRIs and SNRIs are commonly prescribed to treat depression, which is why they’re called “antidepressants.” But mental health professionals sometimes also prescribe them for anxiety disorders.

In addition, according to the Food and Drug Administration (FDA), SSRIs can be prescribed to treat:

Medical professionals generally consider both SSRIs and SNRIs effective for treating depression but measuring just how effective they are has been the subject of debate.

Some antidepressants might be more effective for people than others, which is why doctors will often prescribe one drug and see if it works for you. If it doesn’t help as much as expected, they may prescribe you another instead.

Overall, research has found SSRIs and SNRIs to be equally effective. About 40% to 60% of people with moderate to severe depression who took an antidepressant noticed an improvement in their depression symptoms within 6 to 8 weeks.

In comparison, about 20% to 40% of people noticed an improvement when taking a placebo.

While an antidepressant can begin affecting within a week or two, it usually takes longer for symptoms to truly improve. In some cases, people can feel relief from long-term symptoms or even have symptoms go away altogether.

Both drugs have also been shown to lower the risks of depression recurrence when taken for 1 to 2 years (or longer).

Both SSRIs and SNRIs are generally considered safe, but both can cause some side effects, including:

  • dry mouth
  • sexual disfunction
  • headaches
  • nausea
  • excessive sweating
  • fatigue
  • difficulty sleeping

SSRIs generally have fewer side effects, but in addition to the ones listed above, they can also cause:

  • diarrhea
  • rash
  • weight gain
  • nervousness

In 2004, the FDA added a black box warning to SSRIs because research suggested that they could cause an increased risk of suicidal thoughts and behavior in children and adolescents.

However, studies have explored to what degree this warning was accurate and how it may have turned people in need of help away from SSRIs.

SNRIs can also cause:

  • changes in appetite
  • agitation
  • increased blood pressure
  • increased heart rate
  • heart palpitations
  • difficulty urinating
  • dizziness
  • constipation
  • muscle weakness
  • erectile dysfunction
  • fluid retention
  • tremors

As a result, a doctor might recommend that people with liver problems or high blood pressure avoid SNRIs.

Medical professionals generally avoid recommending SSRIs or SNRIs for pregnant people unless the benefits of taking them outweigh the risks to the pregnant person and baby. Research has suggested SSRIs or SNRIs can pose a risk to the developing fetus.

SSRIs have been shown to increase the risk of developmental concerns for a fetus, including heart and lung issues. SNRIs may cause difficulty breathing, feeding problems, tremors, and other withdrawal symptoms.

Based on data from 2018, more than 13% of U.S. adults 18 years old and older may have used antidepressants in the past 30 days. Many people likely took an SSRI or SNRI, the most commonly prescribed antidepressants in the United States.

Both types of drugs can help treat depression and anxiety by raising critical neurotransmitters in the brain. They’re generally safe, relatively effective, and have fewer side effects than some medications.

These types of antidepressants aren’t for everyone. So, if you find that they aren’t helping you, try not to be discouraged. There may be a better fit for you with another medication.

Living with depression can feel overwhelming. If you feel like you need help, consider talking with a mental health professional or your primary care doctor. They might suggest an SSRI or SNRI if they think it’s appropriate.