Serotonin has a wide variety of functions in the human body. People sometimes call serotonin as happy chemical, because it contributes to well-being and happiness.
The scientific name of serotonin is 5-hydroxytryptamine (5-HT). Serotonin is mainly present in the brain, bowels, and blood platelets.
Serotonin is a neurotransmitter, and some researchers also consider it a hormone. The body uses serotonin to send messages between nerve cells.
Serotonin plays a role in mood, emotions, appetite, and digestion. As the precursor for melatonin, serotonin helps regulate sleep-wake cycles and the body clock.
Many investigations have looked at serotonin and what it does, but there is still a lot for us to learn.
In this article, we explain the role of serotonin in the body, uses of drugs that affect serotonin, side effects and symptoms of serotonin deficiency, and how to boost serotonin levels.
What is serotonin?
Serotonin is a result of tryptophan combining with tryptophan hydroxylase. Tryptophan is a component of proteins, and tryptophan hydroxylase is a chemical reactor. Together, they form 5-HT, or serotonin.
The intestines and the brain produce serotonin. Serotonin is also present in blood platelets and plays a role in the central nervous system (CNS).
Occurring throughout the body, serotonin influences a range of physical and psychological functions.
Serotonin is also present in animals, plants, and fungi. For this reason, some people have looked at food as a source of serotonin.
Serotonin cannot cross the blood-brain barrier. This means that the brain must produce any serotonin that it needs to use. Treatments for depression and other mental health issues do not supply serotonin directly, but cause reactions that can boost serotonin levels in the brain.
However, research suggests that sources of serotonin in other areas, such as the digestive system, may work independently of serotonin in the brain. This findings can suggest the treatment and prevention method for various physiological conditions, such as bone degeneration. 
Function of serotonin
As a neurotransmitter, serotonin relays signals between nerve cells and regulates their intensity.
Scientists believe serotonin plays a role in mood and the CNS, and affects functions throughout the body. 
Serotonin may have an impact on:
- bone metabolism
- cardiovascular health
- eye health
- blood clotting
- neurological disorders
However, the relationship between serotonin and many bodily functions remains unclear.
Serotonin and depression and other health conditions
Scientists do not know precisely what causes depression, but one theory says that depression is caused by an imbalance of neurotransmitters in the body. 
Doctors usually prescribe selective serotonin reuptake inhibitors (SSRI) as antidepressants. Fluoxetine (Prozac) is one example.
Normally, the body reabsorbs a neurotransmitter after it has transmitted its neural impulse. SSRI stops the body from reabsorbing serotonin, leaving higher levels of serotonin to circulate.
Many people find that SSRI help relieve their symptoms, although the link between depression and serotonin remains unclear.
One problem for researchers is that, while they can measure serotonin levels in the bloodstream, they cannot measure its levels in the brain.
As a result, they do not know whether serotonin levels in the bloodstream reflect the serotonin level in the brain. It is also impossible to know whether SSRI can really affect the brain.
Mouse studies have produced conflicting evidence. Some studies suggest that increasing serotonin levels can help reduce stress and depression, but other studies indicate that serotonin levels make no difference. 
In 2015, one study called the use of SSRI to treat depression as a misconception. 
Nevertheless, while scientists have not yet proven the serotonin theory of depression, SSRI is helping many people.
Apart from depression, doctors may prescribe drugs that regulate serotonin levels to treat a number of other disorders, including:
- bipolar disorder
- post-traumatic stress disorder
- obsessive-compulsive disorder
- panic disorders
As with depression, some scientists have questioned whether serotonin is the only factor impacting these disorders. 
Selective serotonin reuptake inhibitors (SSRI)
SSRI increases serotonin levels by preventing the body from reabsorbing serotonin neurotransmitters. Serotonin levels remain high in the brain, and this condition may elevate a person’s mood.
SSRI that have approval from the Food and Drug Administration (FDA) in the US for treating depression are:
- citalopram (Celexa)
- escitalopram (Lexapro)
- paroxetine (Paxil, Pexeva)
- sertraline (Zoloft)
- vilazodone (Viibryd)
Adverse effects of SSRI
SSRI has some side effects, but these side effects usually improve over time.
The side effects include:
- nausea and vomiting
- restlessness and agitation
- diarrhea or constipation
- body weight loss or appetite loss
- increased sweating
- blurred vision
- sleepiness or insomnia
- feeling shaky
- dry mouth
- low sex drive
- erectile dysfunction
- suicidal thoughts
In some cases, there may be other side effects like:
- bruising or bleeding easily
- body stiffness or shaking
- difficulty urinating
In rare cases, taking too much of a drug that boosts serotonin levels or combining two such drugs can lead to serotonin syndrome. This is a potentially life threatening condition that may require emergency treatment.
SSRI and suicide
A person who uses SSRI to treat depression will not experience the benefits at once. At first, symptoms may worsen before improving. Anyone experiencing thoughts of suicide should seek help at once.
The FDA in the US requires all antidepressants to carry a black box warning about the danger of suicide during the initial stages of treatment, especially in people aged under 25 years.
Serotonin deficiency symptoms
According to the National Institute on Drug Abuse (NIDA) in the US, low levels of serotonin can lead to memory problems and a low mood. 
These are symptoms of depression, although scientists have not confirmed a link between low serotonin levels and depression.
NIDA notes that when people use certain recreational drugs, such as MDMA (ecstasy), the body releases large amounts of serotonin.
This action can lead to serotonin depletion and a low mood, confusion, and other symptoms that last several days.
Animal studies have suggested that these drugs may damage the nerves that contain serotonin, with long-lasting adverse effects.
How to boost serotonin levels
Some natural remedies may help boost serotonin levels in the body. 
These natural remedies include:
- practicing meditation
- having light treatment, already in use for treating seasonal affective disorder
- doing regular exercise
- consuming foods that are high in tryptophans
Foods which can help increase serotonin level
Tryptophan is an amino acid which is present in some foods. Some research has linked intake of dietary tryptophan to more positive mood scores, possibly because tryptophan bolsters serotonin levels. 
Foods that contain tryptophan include:
- soy products
- talbina, a dish made from barley
The body uses tryptophan to create serotonin. Eating foods that contain tryptophan may help support this process, but it does not mean that the body will necessarily absorb and use the tryptophan. In addition, the amount of tryptophan in foods may be too low to make a difference.
Bananas contain serotonin, but bananas can only improve a person’s mood if the serotonin in the bananas will reach the brain. This does not happen.
In one study, a number of older people improved their scores on cognitive tests after taking tryptophan supplements for 12 weeks.
People should speak to a doctor before using any supplements, in case there is a risk of adverse effects.
Serotonin and the gut-brain axis
Scientists are becoming more interested in the idea that gut microbiota might influence the nervous system — including behavior, mood, and thinking — through a link known as the gut-brain axis. 
If that idea is true, serotonin could provide the crucial link. This means that diet and the gut microbiota could play a role in preventing and treating conditions such as anxiety and depression.
Serotonin, or the happy chemical, plays a role in various physical and psychological functions.
SSRI are drugs that affect serotonin levels. These drugs can help manage the symptoms of depression, although experts are still unsure exactly how it works.
Anyone considering taking a drug or supplement that affects serotonin levels should consult their doctor first to ensure it is safe for them to use.