Table 1:

Steady-state concentrations (Css) of Famotidine and Cimetidine at standard doses compared to the half maximal inhibitory concentration (IC₅₀) value of Famotidine or Cimetidine for histamine H₂ receptor antagonism

IC50 or Css	Concentration (mass/volume)	Concentration (molarity)
Famotidine
IC50 Histamine H2	13 mg/L	0.039 mM
IC50 Neutrophil H2 O2− assay	67 mg/L	0.201 mM
IC50 Neutrophil H2 cAMP assay	8 mg/L	0.024 mM
IC50 Eosinophil H2 cAMP assay	53.6 mg/L	0.158 mM
IC50 Mast Cell H2 cAMP increase	Not determined
	Total Concentration (mass/volume)	Total Concentration (mass/volume)	Free drug Concentration3 (mass/volume)	Free drug Concentration3 (molarity)
Css (20 mg tablet p.o. q.d.)	17.7 mg/L	0.053 mM	14.2 mg/L	0.042 mM
Css (20 mg capsule p.o. q.d.)	18.4 mg/L	0.055 mM	14.7mg/L	0.044 mM
Css (20 mg tablet p.o. b.i.d.)	35.4 mg/L	0.105 mM	28.3 mg/L	0.084 mM
Css (20 mg capsule p.o. b.i.d.)	36.8 mg/L	0.109 mM	29.4 mg/L	0.087 mM
Css (20 mg tablet p.o. t.i.d.)	53.1 mg/L	0.157 mM	42.5 mg/L	0.126 mM
Css (20 mg capsule p.o. t.i.d.)	55.3 mg/L	0.164 mM	44.2 mg/L	0.131 mM
Css (40 mg tablet p.o. t.i.d.)1	55.4 mg/L	0.164 mM	44.3 mg/L	0.131 mM
Css (40 mg tablet p.o. t.i.d.)2	80.8 mg/L	0.239 mM	64.6 mg/L	0.192 mM
Css (60 mg tablet p.o. t.i.d.)	144.3 mg/L	0.425 mM	115.4 mg/L	0.340mM
Css (120mg IV every 8 hours)	1,290 mg/L	1.092 mM	1,032 mg/L	0.874 mM
Cimetidine
IC50 Histamine H2	400–780 mg /L	1.59–3.09 mM
Css (200 mg tablet p.o. q.d.)	175 mg /L	0.69 mM	140 mg/L	0.055 mM
Css (300 mg tablet p.o. q.d.)	226 mg /L	0.90 mM	180.1 mg/L	0.720 mM

¹calculated using pK data reported by Lin et al 1987 ³⁹

²calculated using pK data reported by Yeh et al 1987 ⁴⁰

³Both famotidine and cimetidine are approximately 20% protein bound in systemic circulation ^51,52