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Hydrochloric Acid (ATC A09AB03): Toxicological and Lactological Monograph

Active substance: Hydrochloric acid (hydrogen chloride in aqueous solution)
ATC code: A09AB03
ATC subgroup: A09AB — Acid preparations
Therapeutic group: Digestives, including enzymes
CAS number: 7647-01-0
Molecular formula: HCl
Molecular weight: 36.46 g/mol

ATC class and general pharmacology

Hydrochloric acid is classified in the Anatomical Therapeutic Chemical (ATC) system as A09AB03, within A09AB (acid preparations), A09A (digestives, including enzymes), and A09 (digestives, including enzymes). The WHO ATC/DDD Index does not assign a defined daily dose (DDD) to A09AB03. Hydrochloric acid is also recognized in pharmaceutical substance databases under CAS 7647-01-0.

Hydrochloric acid is a strong inorganic acid and represents a normal physiological constituent of gastric juice. Gastric hydrochloric acid establishes the highly acidic intragastric environment required for normal digestive physiology. Its principal physiological functions include conversion of pepsinogen to pepsin, maintenance of an appropriate pH for peptic proteolysis, facilitation of protein denaturation, and contribution to the antimicrobial barrier of the stomach.

Historically, diluted hydrochloric acid preparations were administered orally as gastric acidifying agents in patients with hypochlorhydria or achlorhydria. Such therapeutic use is now uncommon and has largely historical significance. Hydrochloric acid nevertheless remains formally represented in the ATC classification as an acid preparation.

From a pharmacokinetic perspective, hydrochloric acid differs fundamentally from conventional organic drug molecules. In aqueous biological environments it dissociates essentially into hydrogen ions (more precisely hydronium species) and chloride ions. Consequently, conventional concepts such as plasma protein binding, apparent volume of distribution, metabolic biotransformation, elimination half-life, or parent-drug renal clearance have limited relevance to orally administered hydrochloric acid. Hydrogen chloride rapidly dissociates into hydrogen and chloride ions.

Following appropriately diluted oral administration, the pharmacological effect is predominantly local within the gastric lumen. Acid is subsequently buffered and neutralized by ingested food, gastrointestinal secretions, bicarbonate entering the proximal intestine, and other physiological buffering systems. Chloride becomes part of the body's normal electrolyte pool, whereas the administered proton load is incorporated into physiological acid-base homeostasis.

This distinction between appropriately diluted pharmaceutical exposure and concentrated hydrochloric acid exposure is essential. The latter represents a corrosive toxicological exposure rather than an extension of the intended pharmacological action.

Toxicological profile

The toxicological properties of hydrochloric acid are dominated by its concentration-dependent corrosive action rather than by conventional systemic organ toxicity. Hydrogen chloride and concentrated hydrochloric acid are highly irritating or corrosive to tissues with which they come into direct contact.

Acute toxicity

Oral exposure to concentrated hydrochloric acid may produce severe chemical injury of the lips, oral cavity, pharynx, esophagus, and stomach. Clinical manifestations can include immediate burning pain, hypersalivation, thirst, dysphagia, odynophagia, nausea, vomiting, hematemesis, abdominal pain, and gastrointestinal hemorrhage. Severe poisoning can result in mucosal necrosis, esophageal or gastric perforation, peritonitis, circulatory collapse, shock, and secondary renal or hepatic injury. Later complications include esophageal or gastric scarring, stricture formation, and gastric outlet obstruction.

A substantial chloride load following severe ingestion has occasionally been associated with hyperchloremia and metabolic acid-base disturbances. Such systemic abnormalities should be distinguished from the predominant mechanism of acute injury, which remains direct corrosive tissue damage.

Inhalation of hydrogen chloride gas or concentrated hydrochloric acid aerosol causes concentration-dependent irritation of the respiratory tract. Manifestations may range from cough, throat irritation, and bronchospasm to laryngeal edema, respiratory distress, pulmonary edema, and, following very high exposure, asphyxia. Reactive airways dysfunction syndrome (RADS) may occur following substantial irritant exposure.

Direct ocular exposure may cause severe pain, conjunctival injury, corneal damage or ulceration and, in severe cases, permanent visual impairment. Concentrated solutions can similarly produce deep chemical burns of the skin and mucous membranes.

Chronic toxicity

Repeated occupational inhalational exposure to hydrogen chloride or acid mist has been associated with chronic respiratory irritation, bronchial inflammation, nasal ulceration, impairment of pulmonary function, irritation of the eyes and skin, and erosion or discoloration of dental enamel.

Hydrogen chloride has not been established as a human systemic carcinogen. Available evaluations have not provided an adequate basis for classification of hydrochloric acid itself as carcinogenic to humans.

Management of significant exposure

There is no specific antidote for hydrochloric acid or hydrogen chloride poisoning; management is primarily decontaminative and supportive. Following significant ingestion of concentrated acid, vomiting should not be induced and attempts at chemical neutralization should be avoided because of the possibility of additional tissue injury. Activated charcoal has no useful role in binding hydrochloric acid. Significant caustic ingestion requires urgent medical assessment, with airway management, supportive treatment and specialist evaluation of gastrointestinal injury as clinically indicated.

The toxicology of accidental or occupational exposure should not be extrapolated directly to correctly formulated medicinal or dietary acidification. Toxicity depends strongly on concentration, quantity, route of exposure, contact time, and buffering conditions.

Lactation pharmacology and infant exposure

Direct clinical pharmacokinetic studies of medicinal hydrochloric acid during human lactation are essentially absent. ATSDR specifically notes that data concerning maternal transfer of hydrogen chloride into breast milk have not been located. Therefore, a quantitative milk-to-plasma ratio, relative infant dose (RID), milk elimination half-life, and absolute infant dose cannot be calculated from clinical lactation studies.

Nevertheless, the physicochemical and physiological behavior of hydrochloric acid permits an important mechanistic assessment.

Hydrochloric acid does not persist in vivo as a conventional intact xenobiotic molecule. In aqueous biological media it dissociates into hydrogen and chloride ions. Following an appropriately diluted oral medicinal dose, most of the acidifying effect occurs within the gastrointestinal lumen, where the administered acid is subjected to extensive buffering and neutralization before any components enter systemic physiological pools.

Consequently, transfer of pharmacologically meaningful amounts of intact hydrochloric acid from maternal plasma into human milk is not biologically expected in the manner characteristic of conventional systemically distributed drugs. Instead, any absorbed chloride contributes to the normal maternal chloride pool, while hydrogen ions are regulated through systemic acid-base homeostasis.

Chloride itself is a normal physiological constituent of human milk and an essential electrolyte. EFSA has cited an average chloride concentration in human milk of approximately 400 mg/L, illustrating that chloride exposure through breast milk is a normal component of infant nutrition. This endogenous milk chloride should not be interpreted as evidence of transfer of intact medicinal hydrochloric acid.

Accordingly, the classical relative infant dose (RID) paradigm is poorly suited to hydrochloric acid. There is no meaningful maternal plasma concentration of persistent intact HCl from which a milk-to-plasma relationship could normally be derived. The clinically relevant maternal determinants are instead the administered concentration and dose, the degree of gastrointestinal neutralization, maternal acid-base status, and whether exposure has produced local or systemic toxicity.

No controlled studies have established adverse effects in breastfed infants following maternal therapeutic use of appropriately diluted hydrochloric acid. Conversely, absence of such reports should not be interpreted as proof of safety because the lactation literature for medicinal hydrochloric acid is extremely limited.

The situation is substantially different after acute poisoning or major occupational exposure. Severe maternal exposure can cause extensive corrosive injury, respiratory compromise, metabolic disturbances, shock, or other systemic consequences. Under those circumstances, management of breastfeeding is secondary to stabilization of the mother, and lactation decisions should be individualized according to maternal condition, concomitant treatments, and the possibility of external chemical contamination.

Lactological safety assessment and clinical recommendations

The available evidence for hydrochloric acid during breastfeeding is characterized by very limited direct lactation data but low mechanistic plausibility of clinically relevant exposure to intact hydrochloric acid through human milk after appropriate therapeutic use.

ATSDR reports an absence of data regarding transfer of hydrogen chloride into breast milk. Thus, a formal evidence-based RID cannot be provided. This limitation should be explicitly acknowledged rather than interpreted as evidence either of significant risk or of demonstrated safety.

From a pharmacological perspective, however, hydrochloric acid undergoes immediate ionic dissociation and physiological neutralization, and chloride is already a normal component of maternal extracellular fluid and human milk. These properties make clinically significant exposure of a breastfed infant to intact hydrochloric acid through milk unlikely after appropriately diluted maternal oral use.

For a breastfeeding patient receiving a legitimate, appropriately formulated hydrochloric acid preparation at a therapeutic dose, routine interruption of breastfeeding would not ordinarily be expected to be necessary solely because of the physicochemical properties of hydrochloric acid. This conclusion is mechanistic rather than based on dedicated clinical lactation trials.

Several qualifications are important:

1. Concentrated hydrochloric acid must never be regarded as equivalent to a therapeutic diluted preparation. Concentrated material is strongly corrosive and can cause life-threatening injury.
2. Because medicinal hydrochloric acid has limited contemporary therapeutic use, the indication for treatment should be reviewed. Where an established and better-characterized therapeutic alternative exists, particularly during breastfeeding, it may be preferable on clinical rather than lactational-pharmacokinetic grounds.
3. Maternal gastrointestinal symptoms such as severe epigastric or retrosternal pain, dysphagia, persistent vomiting, hematemesis, or evidence of mucosal injury require prompt medical assessment.
4. After accidental ingestion of concentrated hydrochloric acid, inhalational poisoning, or extensive dermal exposure, the situation should be treated as an acute toxicological emergency rather than as a routine medication-and-breastfeeding question.
5. In occupational exposure, external contamination of the mother's skin, clothing, breast, or equipment must be eliminated before direct infant contact or milk expression. This is principally a contact-contamination issue rather than a concern about conventional drug secretion into milk.
6. Severe poisoning accompanied by maternal hemodynamic instability, metabolic acidosis, respiratory compromise, extensive gastrointestinal injury, or intensive-care treatment requires individualized lactation management. Any temporary interruption of direct breastfeeding in this context may be dictated by maternal clinical status, contamination risk, procedures, or concomitant medications rather than by demonstrated secretion of hydrochloric acid into milk.

Overall lactological assessment

Direct evidence: Very limited; no adequate human milk pharmacokinetic studies are available.

Milk transfer: Transfer of intact hydrochloric acid in clinically meaningful quantities is mechanistically unlikely because of immediate dissociation, buffering, and physiological regulation.

Relative infant dose: Not established and not meaningfully calculable using conventional drug-lactation methodology.

Infant toxicity through breast milk: No specific toxicity signal has been established, but dedicated clinical studies are lacking.

Breastfeeding compatibility: Appropriately diluted, medically indicated therapeutic use is unlikely to result in clinically important infant exposure through breast milk. Clinical decisions should nevertheless account for the formulation, concentration, maternal dose, indication, maternal condition, and the paucity of direct lactation evidence.

Accidental or concentrated exposure: Requires urgent toxicological assessment; recommendations for routine therapeutic exposure must not be extrapolated to poisoning.

The overall benefit–risk assessment should therefore distinguish sharply between controlled medicinal exposure, for which infant exposure through milk is expected to be negligible from a mechanistic standpoint, and concentrated or accidental exposure, where severe maternal corrosive toxicity and external contamination are the dominant clinical concerns.