Can you tell me about taking tagamet 300 (cimetidine)?

An old acid blocker: Tagamet (cimetidine), Zantac (ranitidine), Pepcid (famotidine) are H2-receptor blockers, introduced by prescription in late 70's/early 80's. All reduce acid production dose-dependently, & promote drug tolerance (requiring increasing doses of drug to block increasing # of receptors with chronic use). OTC dosing is a fraction of original prescription doses. Similar side effects, efficacy, duration.