Systemic : Sodium bicarbonate Nonsystemic : Calcium carbonate Aluminium hydroxide gel Magnesium carbonate/hydroxide/trisilicate Magaldrate These are alkaline substances which neutralize gastric acid and raise the pH of gastric contents. They do not decrease acid production. Sodium bicarbonate: It is water soluble, acts istantaneously but the duration of action is short. Indicated for casual heart burn. Systemic absorption induces alkalosis & increased sodium load may worsen CHF and edema. Contraindicated in cardiac disease, hypertension. Calcium Carbonate : It is a potent and rapidly acting neutralizer which slowly liberates CO2 in stomach, causing distention and discomfort. When taken in large quantities with milk may cause hyper- calcaemia & alkalosis leading to headache, anorexia, weakness, abdominal discomfort, abnormal Ca deposits & renal stone. Calcium carbonate is constipating in most individuals but may cause loose motions in some. Aluminium hydroxide gel : It is a weak and slowly acting antacid & neutralizing action is markedly slowed by food in stomach. It frequently causes constipation, intestinal obstruction can occur in elderly patients. It is a demulcent and has been claimed to coat and protect the ulcercrater. Hypophos- phatemia occurs on chronic use which may cause osteomalacia, but this effect is of use in CRF patients. Magnesium salts : All Mg salts have a laxative action. Mag. hydroxide is poorly water soluble; its suspension (milk of magnesia) has low concentration of OH- ions, and thus low alkalinity. However, it promptly reacts with Hcl & is an efficacious antacid. Mag. Carbonate is more soluble but is crystalline & reacts with Hcl at a slower rate; CO2 is evolved but very gradually. Mag. trisilicate has the lowest solubility & reactivity. Silica produced by reaction with Hcl is gelatinous. It is claimed that mag. trisilicate & silica adsorb &inactivate pepsin & protect the ulcer base. Magaldrate is a hydrated complex of hydroxy magnesium aluminate. It initially reacts rapidly with acid and releases alum. hydroxide which then reacts more slowly. The freshly released alum. hydroxide is in the impolymerized, more reactive form. It is a good antacid with prompt & sustained neutralizing action. Mostly a combination of two or more antacids is used the constipating & laxative effect of Aluminium Magnesium Salt can neutralise each other. Certain antacid combinations are available with Local Anaesthetic compounds to give faster symptomatic relief. Similarly some contain adsorbent, antiflatulent compounds like METHYL POLYSILOXANE (Dimethyl polysiloxane; Simethicone, dimethicone) which is a silicone polymer, a viscous amphiphilic liquid which reduces surface tension & collapses froth (anti foaming-agent.) There are some antacids which are available as SODIUM FREE to be safe for use in hypertensive/uraemic patients, others may contain SORBITOL which makes it safer in diabetic atients. Similarly some are magnesium free i.e. containing only Aluminium compounds thereby chelating phosphate content in diet, so that there is less calcium & phosphate loss in urine in uraemic/osteomalacia patients.