Understanding Depression Medication: What Are SSRIs And SNRIs?

About the Author

Cassie O’Connor is a Senior Pharmacist at Medmate

Low serotonin levels cause depression

While the causes of depression are multifactorial, it is generally accepted that the chemical messenger (neurotransmitter) serotonin plays a role. Serotonin is your body’s natural feel-good chemical. Normal serotonin levels help you to feel emotionally stable, focused, happy and calm. Low serotonin levels can be associated with depression.  

Antidepressant medications generally act by increasing serotonin levels in the brain. They may also affect the levels of other neurotransmitters involved in mood regulation.  

What are the types of antidepressants?

The type of antidepressant your doctor will prescribe for you will depend on you as an individual. Your doctor will consider your medical history, age, individual symptoms of depression and other factors such as if you are pregnant or breastfeeding for females.  

Of the antidepressants most commonly prescribed, the selective serotonin reuptake inhibitors (SSRIs) are often a doctor’s first choice. They include sertraline, citalopram, paroxetine, fluoxetine and fluvoxamine. The SSRIs work by increasing the levels of serotonin in the brain. They do this by blocking the reuptake of serotonin into brain nerve cells (neurons). This makes more serotonin available and improves message transmission between neurons. SSRIs are generally well tolerated and non-sedating, making them a good first line choice for most types of depression. 

For some people, the first antidepressant they try will have a positive effect on their symptoms of depression and the side effects will be minimal. Others may have to trial more than one before they find the antidepressant that suits them best.  

The selective serotonin and noradrenaline reuptake inhibitors (SNRIs) are one such second line antidepressant. SNRIs include venlafaxine, desvenlafaxine and duloxetine. These work by increasing the levels of both serotonin and noradrenaline in the brain. They can also help with anxiety disorders and chronic pain in addition to depression.  

If insomnia is a predominant symptom, an antidepressant that helps with sleeping issues may be appropriate. The noradrenaline-serotonin specific antidepressants (NaSSAs) such as mirtazapine are a relatively new class of antidepressants. They are helpful when there are problems with anxiety or sleeping.  

Other classes of antidepressants include the tricyclic antidepressants (TCAs), noradrenaline reuptake inhibitors (NARIs) even some atypical standalone medications such as agomelatine.  

It can take a while to see improvements when starting an antidepressant. In general, antidepressants take at least 2 weeks before they start to help and 4-6 weeks before depression symptoms begin to improve.