Fenfluramine Side Effects

Medically reviewed by Drugs.com. Last updated on Feb 2, 2025.

Applies to fenfluramine: oral solution.

Side effects include:

Decreased appetite, somnolence, sedation, lethargy, diarrhea, constipation, abnormal echocardiogram, fatigue, malaise, asthenia, ataxia, balance disorder, gait disturbance, increased BP, drooling, salivary hypersecretion, pyrexia, upper respiratory tract infection, vomiting, decreased weight, falls, status epilepticus.

For healthcare professionals

Applies to fenfluramine: oral solution, oral tablet.

General adverse events

The more commonly reported adverse effects have included decreased appetite and weight, somnolence, sedation, lethargy, diarrhea, constipation, increased blood pressure, gait disturbances, balance disorder, pyrexia, asthenia, vomiting, fall, and status epilepticus.^[Ref]

Cardiovascular

• Very common (10% or more): Abnormal echocardiogram (up to 23%), increased blood pressure (up to 13%)
• Common (1% to 10%): Increased heart rate^[Ref]

Abnormal echocardiogram included trace and mild mitral regurgitation, and trace aortic regurgitation, which are considered physiologic. Valvular heart disease and pulmonary arterial hypertension were evaluated in the clinical studies via echocardiography for up to 3 years in duration and no patient developed findings consistent with either condition. During the open-label extension study, trace mitral regurgitation and trace aortic regurgitation were reported in 14% and 0.4%, respectively. Trace and mild mitral regurgitation, and trace aortic regurgitation are

considered physiologic in the absence of structural valve abnormalities.^[Ref]

Nervous system

• Very common (10% or more): Somnolence, sedation, and lethargy (26%)
• Common (1% to 10%): Ataxia, balance disorder, gait disturbance, hypotonia, falls, headache, status epilepticus, stereotypy, tremor^[Ref]

Metabolic

• Very common (10% or more): Decreased appetite (up to 49%), decreased weight (up to 13%)
• Common (1% to 10%): Dehydration, decreased blood glucose^[Ref]

This drug decreases appetite and causes dose-related weight loss. In clinical studies, decreased appetite and weight were reported in 37% and 9%, respectively (placebo=8% and 1%). Weight loss of 7% or greater from baseline was reported in 19% of patients treated with this drug compared to 2% of patients on placebo.^[Ref]

Psychiatric

• Common (1% to 10%): Abnormal behavior, insomnia, irritability, negativism
• Postmarketing reports: Aggression^[Ref]

Genitourinary

• Common (1% to 10%): Urinary incontinence, enuresis, urinary tract infection^[Ref]

Respiratory

• Very common (10% or more): Upper respiratory infections (up to 21%)
• Common (1% to 10%): Bronchitis, croup, rhinitis, laryngitis^[Ref]

Other

• Very common (10% or more): Fatigue, malaise, and asthenia (up to 30%), pyrexia (up to 21%)
• Common (1% to 10%): Chills, decreased activity, contusion, ear infection^[Ref]

Dermatologic

• Common (1% to 10%): Rash, eczema^[Ref]

Endocrine

• Common (1% to 10%): Blood prolactin increase^[Ref]

Gastrointestinal

• Very common (10% or more): Diarrhea (up to 31%), drooling and/or salivary hypersecretion (up to 13%)
• Common (1% to 10%): Constipation, gastroenteritis, vomiting^[Ref]

Musculoskeletal

• Common (1% to 10%): Hypotonia^[Ref]

Ocular

• Very common (10% or more): Glaucoma^[Ref]

See also:

Frequently asked questions

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Further information

Fenfluramine side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Note: Medication side effects may be underreported. If you are experiencing side effects that are not listed, submit a report to the FDA by following this guide.

Medical Disclaimer