3 Clinical psychopharmacology3.4 Therapeutics
3.4.5.3 Eating disorders
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Written by Resident Doctor Committee · Published
Pharmacokinetics in malnutrition
Change | Effect on medicines |
|---|---|
Less tissue mass, dehydration | Altered distribution; some levels rise |
Low albumin | More free drug, so total levels mislead |
Gastroparesis, constipation | Delayed or variable absorption |
Vomiting after a dose | Uncertain retained dose |
Severe starvation | Altered hepatic metabolism |
Poor renal perfusion | Reduced renal clearance |
There is no single weight-based dose correction, because effects vary.
A normal body mass index does not exclude serious physical instability.
Physical risks
Restriction causes bradycardia, hypotension and reduced cardiac reserve.
Vomiting commonly causes hypokalaemic alkalosis, and low magnesium makes potassium replacement less effective.
Bazett's formula can under-correct the QT interval in bradycardia, so consider Fridericia's formula and expert interpretation.
Syncope, arrhythmia, chest pain or marked instability needs urgent medical assessment.
Lithium levels can rise with dehydration or sodium depletion even when the dose is unchanged.
Creatinine-based measures overestimate renal function when muscle mass is low.
Bupropion is contraindicated with current or previous anorexia nervosa or bulimia nervosa because of seizure risk; the label does not cover every eating disorder.
Anorexia nervosa
Medication is never the sole treatment, and starvation itself causes low mood, food preoccupation, rigidity and cognitive symptoms.
Olanzapine gives a modest average weight gain without consistent improvement in core psychological symptoms.
Bulimia nervosa and binge eating disorder
Eating-disorder-focused psychological treatment comes first in both.
Aspect | Bulimia nervosa | Binge eating disorder |
|---|---|---|
Drug with evidence | Fluoxetine, usually 60 mg a day | Lisdexamfetamine |
Licence | UK, as adjunctive treatment | US adults, not the UK |
Cautions | Cardiac, electrolyte and overdose risk | Misuse, appetite suppression, cardiovascular effects |
Outcome measure | Binge and purge frequency, function | Binge frequency and function, not weight loss alone |
The benefit of fluoxetine cannot be assumed for other antidepressants.
Monitoring
Monitoring frequency follows instability, and one normal result does not establish safety.
Ask sensitively about stimulants, laxatives, diuretics, thyroid medicines and insulin omission, which can be life-threatening in type 1 diabetes.
Refeeding
Refeeding shifts phosphate, potassium and magnesium into cells and raises thiamine demand.
Severe hypophosphataemia can impair cardiac, respiratory and neurological function.
A specialist plan prevents both refeeding complications and underfeeding, with thiamine and frequent early biochemical monitoring.
Reassess doses during recovery, because distribution and clearance change again.