The goal of this clinical medicine study is to investigate how does antidepressant fluoxetine modulate anger processing in healthy young people . The main questions it aims is to answer are:

1. How does fluoxetine affect responses to anger-related stimuli such as words, faces, and autobiographical recall?
2. How does fluoxetine influence responses during frustration induction in frustrative non-reward and threat paradigms?
3. Does the effect manifest in physiological markers, including heart rate variability and facial expressions?

Researchers will compare fluoxetine to a placebo to see if drug fluoxetine affects anger processing.

Participants will:

Take 20mg fluoxetine or a placebo every day for 7 days. Visit the university site for questionnaire and tasks assessments. Heart rate variability and facial expressions will be recorded in some of the tasks.

Inclusion Criteria:

• Be aged 18-24 years (inclusive)
• Be resident in the UK for the duration of the study
• Have normal or corrected to normal vision
• Participant is willing and able to give informed consent for participation in the research
• Sufficiently fluent English to understand and complete the study

Exclusion Criteria:

The participant may not enter the study if ANY of the following apply:

Psychiatric History:

• Current or past diagnosis of any psychiatric disorder, as determined by the Structured Clinical Interview for the DSM-5 (SCID-5) and self-report. This includes, but is not limited to, depression, anxiety disorders, alcohol or drug dependency, personality disorders, suicidal ideation, and other psychiatric conditions;
• First degree relative with a diagnosis of mania;

Lifestyle:

• Heavy smoker or vaper (> 10 cigarettes per day, or >2 mL e-liquid, or >15mg/day from a nicotine patch);
• Heavy use of caffeine (drink > 4 of 250ml cups/cans of coffee or energy drinks per day);
• Heavy drinker (drink >14 standard alcoholic drinks per week);
• Current or recent use (in the last 3 months) of any psychoactive substance according to self-report and a urine drug test screening for recent use of 10 common recreational substances;

Physical Health:

• Severely underweight or overweight in a manner that renders them unsuitable for the study in the opinion of the study medical advisor;
• Known contraindication to fluoxetine, such as hypersensitivity to fluoxetine or any component in its formulation;
• Pregnancy, as determined by a urine pregnancy test or plans to become pregnant within the next 3 months;
• Breastfeeding;
• Not able to consume gelatine;

Medical History:

• Past or ongoing health issue which, in the opinion of the study medical advisor, may interfere with the safety of the participant or the scientific integrity of the study, including but not limited to: seizures or epilepsy, heart rhythm problem, renal disease, hepatic disease, glaucoma, diabetes, bleeding disorders or clotting conditions (e.g., haemophilia, thrombocytopenia);
• Diagnosis of a significant neurological condition (e.g., epilepsy, multiple sclerosis, traumatic brain injury).
• Diagnosis of a neurodevelopmental condition, such as autism spectrum disorder (ASD) or attention-deficit/hyperactivity disorder (ADHD);
• Current or recent use of medication that might interfere or interact with the effects of fluoxetine, in the opinion of the study medical advisor, including but not limited to: monoamine oxidase inhibitors (MAOIs), medications affecting serotonin levels (e.g., tramadol, triptans, St. John's Wort), anticoagulants or blood thinners (e.g., warfarin), anti-inflammatory medications (e.g., aspirin, ibuprofen), or medications known to affect heart rhythm;

Prior Study Participation:

• Participation in any other psychological or medical experiment involving taking any kind of drug/medication/vaccine, within the last 3 months;
• Participation in any other study involving the current or similar tasks, within the last 6 months;

Others

•Any other significant finding which may arise during the screening process and which, in the opinion of the Principal Investigator/medical advisor, may influence the scientific integrity of the study, or the participant's ability to participate in the study.

Data which has been fully de-identified and relevant supporting materials (e.g., study protocol, data dictionary, and analysis documentation) may be shared with other academic organisations in the future, including those outside of the UK and the EU. Participants will be informed of this and specific consent to this is obtained within the Informed Consent Form.

Data will be available in a few months after all data has been completed (ETA Oct 2026), unblinding has occurred (ETA Dec 2026), and all data analyses has been completed (Dec 2027).

Fully de-identified individual participant data (IPD) and relevant supporting documentation may be shared with academic researchers for scientific research purposes. Access will be limited to researchers affiliated with recognised academic or research institutions, including those outside of the UK and EU.

Approved researchers will be granted access only to fully de-identified IPD and relevant supporting materials. Data will be shared via secure transfer methods or through a controlled-access data repository.