Citalopram

Pharmacogenomic (PGx) testing can help predict how you might respond to citalopram, a commonly prescribed SSRI used to treat depression and anxiety. Genetic differences in how your body processes medications can affect both how well the drug works and the likelihood of side effects. Understanding these variations allows for more personalised—and safer—prescribing decisions.

SSRIs Can Interact with Other Medicines and Supplements

If you're taking other medications or supplements—especially those that increase serotonin levels, like tryptophan or St. John’s Wort—it's important to let your GP or pharmacist know. Combining citalopram with these substances can increase serotonin levels too much and may lead to a serious condition known as serotonin syndrome (3).

Increased Suicidal Thoughts in the First 4 Weeks

SSRIs, including citalopram, have been associated with an increased risk of suicidal thoughts during the first four weeks of treatment (2, 10, 11). This is especially important for children, adolescents, and their carers to be aware of. These symptoms usually wear off over time, and the treatment often becomes very effective.

Citalopram Side Effects

SSRIs like citalopram are generally considered safe, but some people may be more prone to serious side effects due to genetic differences. Pharmacogenomic (PGx) testing can help identify individuals at higher risk of rare but important reactions such as serotonin syndrome, prolonged QT interval, or hyponatremia. These complex syndromes are more commonly associated with SSRIs than with older antidepressants, so it’s important to be aware of the signs and seek medical attention if they arise.

Serotonin Syndrome

Serotonin syndrome (5) is a potentially serious and fast-developing condition. It can begin within one hour of starting a new medicine or drug combination in about 30% of cases—or within six hours in 60% of cases. The symptoms relate to changes in brain function, muscle reflexes, and automatic body functions:

• Brain function: Agitation, confusion, anxiety, hypomania, hypervigilance, seizures, or coma

• Muscle reflexes: Tremors, poor coordination, myoclonus (involuntary rhythmic jerks)

• Autonomic functions: Sweating, fast heart rate, diarrhoea, fever, or dangerously high body temperature (over 40°C)

You don’t need to have all of these symptoms to be diagnosed with serotonin syndrome. While some cases are mild, the condition can become life-threatening and must be treated urgently.

Prolonged QT Interval

Citalopram is one of the SSRIs most strongly associated with prolonged QT interval, a heart rhythm disturbance (2, 3). Symptoms may include:

• Palpitations

• Feeling faint or actually fainting

• Light-headedness

• Weakness

• In some cases, seizures

Risk factors for prolonged QT interval (7) include:

• Female sex

• Older age

• Recent conversion from atrial fibrillation using QT-prolonging drugs

• Taking multiple medications that prolong QT interval

• Electrolyte imbalances (e.g. low potassium, magnesium, or calcium)

• Diuretic use

• Liver or kidney dysfunction

• Slow heart rate (bradycardia)

• Genetic predisposition (e.g. silent long QT syndrome)

• Underlying heart disease (heart failure, previous heart attack, or thickened heart muscle)

• Digitalis therapy

If you experience symptoms of a prolonged QT interval, seek immediate medical attention.

Hyponatremia

Hyponatremia (8) is a condition where sodium levels in the blood drop too low, which can happen with SSRIs like citalopram. Symptoms vary depending on how quickly and how severely sodium levels fall.

• Severe cases can cause brain swelling (oedema), leading to vomiting, reduced consciousness, or coma.

• Milder cases may cause mood changes, confusion, or unusual behaviour.

• Even mild hyponatremia can be dangerous and should be addressed quickly.

If you notice any of these symptoms, contact a healthcare provider without delay.

Other Side Effects

Apart from rare but serious side effects, citalopram may also cause more common reactions, which vary from person to person. These can include:

• Anxiety

• Changes in appetite (increase or decrease)

• Joint pain

• Chronic fatigue

• Difficulty concentrating

• Dry mouth

Pharmacogenomic differences may partly explain why some people experience these symptoms more strongly than others.

Your genetic makeup can influence how well citalopram works for you and how likely you are to experience side effects. One key gene involved is CYP2C19, which plays a major role in how citalopram is metabolised in the liver.

• People with reduced CYP2C19 function may process the drug more slowly, leading to higher levels in the body and a greater risk of side effects like QT interval prolongation.

• Those with ultra-rapid metabolism may break the drug down too quickly, which can make it less effective at treating depression or anxiety.

Knowing your CYP2C19 status through pharmacogenomic testing can help your doctor tailor your prescription—choosing the right dose or even a different medication to improve outcomes and reduce risks.

Related Medications:

Other antidepressants are also influenced by genetic variations in CYP2C19 or related enzymes. This includes:

• Escitalopram – another SSRI closely related to citalopram, also influenced by CYP2C19

• Sertraline – an SSRI with moderate dependence on CYP pathways

• Amitriptyline – a tricyclic antidepressant affected by both CYP2C19 and CYP2D6

• Paroxetine – mainly metabolised by CYP2D6, another enzyme involved in antidepressant response

Pharmacogenomic testing can guide safer and more effective choices across this wider group of medications.