Antacids

Mechanism of Action

The antacids reduce the acid reaching the duodenum by neutralizing the acid present in the stomach. The main therapeutic objectives are:

• Alleviating pain
• Relieving pylorospasms
• Avoid digestion and corrosion by acid chyme

The salts' mechanism of neutralization of acid varies, and each salt has a different mechanism with the ultimate goal of acid neutralization.

Aluminum Hydroxide

The formulation of aluminum hydrochloride and water results in the neutralization of the acid in the stomach. It is also known to inhibit pepsin activity.[7] Aluminum hydroxide is complexed with a sulfated polysaccharide sucrose octasulfate to form sucralfate. This complex does not have a significant buffering action against the acid or has no effect on the pepsin secretion, and does not alter the gastric acid production in any way.[8][9] Nevertheless, it is known to heal chronic ulcers and prevent acute mucosal damage induced chemically by reducing access to pepsin and acid. Sucralfate, like its aluminum hydroxide component, is known to stimulate angiogenesis and granulation tissue formation.[8]

Aluminum hydroxide is also useful in hyperphosphatemia due to its ability to bind phosphate in the gastrointestinal (GI) tract and subsequently prevent the absorption of phosphate.[10]

Calcium Salts

Calcium salts neutralize gastric acidity, resulting in increased gastric and duodenal bulb pH; they also inhibit pepsin's proteolytic activity if the pH is greater than 4 and increase lower esophageal sphincter tone. The calcium released from calcium carbonate is known to increase peristalsis in the esophagus, pushing the acid into the stomach and providing relief from heartburn symptoms. The calcium salts also form combined insoluble compounds with dietary phosphate and prevent the absorption of the latter.[11]

The acid-neutralizing mechanism of the antacids is well understood, as mentioned above. In addition to this, other mechanisms add to the ulcer healing properties of this class of drugs. The exact mechanism is still unclear, but it is believed to be a combination of[12][7]:

• Ability to promote angiogenesis
• Bind to bile acids
• Inhibit peptic activity
• Suppress Helicobacter pylori growth

Administration

The dose for antacids depends upon the patient's age, the purpose of administration (neutralization of acid or off-label use), and the presence of other comorbidities like renal or hepatic impairment. As all the forms of these medications are available in over-the-counter formulations, the dosing recommendation varies by product/and or manufacturer. 

 Aluminum Hydroxide (Antacid) 

• Oral: Usually taken daily after meals and at bedtime at a dose of  640 mg up to 5 to 6 times a day with a maximum dose of 3,840 mg per 24 hours.
• Suspension  The dose for suspension form is 320 mg/5 mL (473 mL). The suspension is to be shaken before use and is to be followed by water.

Hyperphosphatemia

• Oral:  Use is reserved for patients with serum phosphorus levels greater than 7 mg/dl. It is to be taken with meals at a dose of 300  to 600 mg three times a day, with use limited to a short period (4 weeks).[13]

Calcium Carbonate (Antacid)

It is used up to a maximum dose of 8000 mg per day (1 to 4 tablets daily) for up to two weeks for symptomatic relief.

Hyperphosphatemia

The total dose is not to exceed 2000 mg per day.[13]

Pregnancy and Breastfeeding

Antacids containing aluminum salts are safe to be used in pregnant women as well as for women during labor for aspiration prophylaxis. The information regarding the use of aluminum-containing antacids in breastfeeding females has not been studied, but aluminum is known to be endogenous to breast milk.[14][15] In the case of calcium-containing antacids, excessive use is to be avoided in pregnant women as calcium crosses the placenta. The amount of calcium reaching the fetus is dependent on the physiological changes in the mother. Maternal calcium intake also affects the amount of calcium excreted in breast milk; the currently prevailing opinion is that the use of calcium-containing antacids is safe during breastfeeding.[16]

Adverse Effects

Adverse effects are prominent in the infant and the elderly populations. The chronic use of antacids in this population is not a recommendation due to safety concerns.

Aluminum Hydroxide

Aluminum use is associated with an increased risk of toxicity in individuals with renal failure and infants. It presents as[17][18][19][20]:

• Osteopenia
• Microcytic anemia
• Neurotoxicity
• Osteomalacia
• Constipation 
• Fecal impaction
• Nausea
• Vomiting
• Abdominal cramps
• Hypomagnesemia
• Hypophosphatemia

Calcium Carbonate

The adverse reactions often seen with this  group of antacids are[20][21]:

• Abdominal pain
• Anorexia
• Constipation
• Acid rebound
• Nausea
• Vomiting
• Flatulence
• Xerostomia
• Headache
• Hypercalcemia
• Hypophosphatemia
• Milk-alkali syndrome

Antacids can exhibit clinically significant interactions with other medication a patient may be taking.[22] Some examples include:

• Using antacids concomitantly with acidic drugs (e.g., digoxin, chlorpromazine isoniazid) can result in impaired absorption of these acidic drugs, reducing the blood concentrations of the drugs and impairing their therapeutic effects.
• Concurrent antacid use with some drugs (e.g., pseudoephedrine, levodopa) can result in increased absorption of the drugs, leading to potential toxicity or adverse events from increased serum concentration of these drugs.
• Antacids containing magnesium trisilicate and magnesium hydroxide can bind to drugs like tetracycline and fluoroquinolone antibiotics, impeding their absorption and therapeutic effects.
• Sodium bicarbonate significantly affects urine acidity, which can alter the renal elimination of some drugs by the kidney; sodium bicarbonate inhibits the excretion of basic drugs such as amphetamines and quinidine while increasing the excretion of acidic drugs like aspirin.

Contraindications

The absolute contraindication is hypersensitivity to any component of the formulation. Also, antacid agents require caution in patients with:

• Renal  failure
• Heart failure
• Edema 
• Cirrhosis
• Low-sodium diets
• Uremia
• GI hemorrhages
• Hyperparathyroidism
• Renal calculus
• Achlorhydria

Monitoring

The average therapeutic dose of antacid is 10 to 15 mL (1 tablespoon or one package content) of liquid or 1 to 2 tablets 3 to 4 times a day. Periodic monitoring of calcium and phosphorus plasma concentrations is a suggested practice in patients on chronic therapy.

Toxicity

No information is available regarding toxicities caused by aluminum- and calcium-containing antacids. However, antacids are to be used cautiously in the high-risk population mentioned above.

Enhancing Healthcare Team Outcomes

Antacids are available as over-the-counter medications, meaning that patients do not require a prescription to obtain them. These medications only provide symptomatic relief, which may mask an underlying disorder, but lack of awareness in this regard may delay the diagnosis in health conditions like GERD, peptic ulcer, gastric ulcer, and hiatal hernia. This situation results in the improper use of these medications with little to no relief of symptoms. There is a need to educate patients regarding the importance of a correct mode of administration, time of administration, and dosage for the prompt and prolonged relief of symptoms. Although these medications do not cause toxicities in high doses, it is imperative to understand their interaction with other medicines, especially in patients engaged in polypharmacy.[23]

Nursing can verify drug use on intake and at each exam and report to the treating clinician. Pharmacists will encounter patients in the retail pharmacy and can counsel and educate patients on the advantages and pitfalls of antacid therapy and inform the patient's treating physician if there are any issues (drug-drug interactions, for example). Even a medication as apparently benign as OTC antacids can significantly affect patient health, and the interprofessional team needs to work together and communicate to achieve positive outcomes. [Level 5]