Activated charcoal

  Common indications

1. A single dose of activated charcoal may be used to reduce absorption of certain poisons (including some drugs in overdose) from the gut.
2. Multiple doses of activated charcoal may also be used to increase the elimination of certain poisons.

 

Mechanisms of action

• Charcoal is ‘activated’ through various processes, including blasting it with steam or hot air. These processes increase the pore size to produce a surface area of around 1000 m2/g. 
• Molecules are adsorbed onto the surface of the charcoal as they travel through the gut, reducing their absorption into the circulation.
• Van der Waals (weak intermolecular) forces are responsible for the adsorption.
• However, activated charcoal is only useful in cases where the poison ingested is likely to be adsorbed onto it. The affinity of a substance for activated charcoal is determined by its ionic status and its solubility in water. 
• Weakly ionic, hydrophobic substances (e.g. benzodiazepines, methotrexate) are generally well adsorbed to activated charcoal. 
• By contrast, strongly ionic and hydrophilic substances (e.g. strong acids/bases, alcohols, lithium and iron) are not adsorbed.
• Activated charcoal can also increase the elimination of certain poisons. This may be useful for substances adsorbed by charcoal that can readily diffuse back into the gut. In this case, multiple doses of activated charcoal can be used to maintain a steep concentration gradient of the poison (high in the circulation, low in the gut), encouraging diffusion out of the circulation and hastening elimination of the drug. This is sometimes referred to as ‘gut dialysis’.

 

Important adverse effects

• Aspiration of activated charcoal can lead to serious complications such as pneumonitis, bronchospasm and airway obstruction.
• It can also precipitate intestinal obstruction.
• However, the most common adverse effects of activated charcoal are black stools and vomiting.

 

Warnings

• Activated charcoal should not be used in patients with a reduced level of consciousness, unless their airway is first protected by endotracheal intubation. 
• Caution is required when prescribing activated charcoal to patients with persistent vomiting, as there is a risk of aspiration. 
• Those with reduced gastrointestinal motility have an increased risk of intestinal obstruction.

 

Important interactions

• Activated charcoal prevents absorption of many drugs taken therapeutically as well as those taken in overdose.