toxic megacolon develops heal completely and
without residual complications. Hirschsprung’s
disease often coexists with other congenital anom-
alies, notably DOWN SYNDROME.
See also GENE; INHERITANCE PATTERNS.
H2 antagonist (blocker) medications Medica-
tions that block molecular structures called hista-
mine 2 receptors in the lining of the STOMACH.
Histamines are chemicals called mediators that
stimulate specific cells. Cells within the stomach’s
lining, called enterochromaffin-like (ECL) cells,
release HISTAMINEin response to the digestive HOR-
MONEgastrin. The histamine binds with H2 recep-
tors on the parietal cells in the stomach. This
binding stimulates the parietal cells to release
hydrochloric acid into the stomach. H2 antago-
nists, or blockers, bind with the H2 receptors as
well, blocking them from binding with endoge-
nous histamine 2. The result is a decrease in acid
production.
Other histamines are involved in different body
functions. Histamine 1 (H1) stimulates smooth
MUSCLE contraction and IMMUNE RESPONSE. H1 is
familiar for its role in allergic response. Histamine
3 (H3) has NEUROTRANSMITTERactivity. Histamine
receptors are primarily unique; only H1 receptors
accept histamine 1, only H2 receptors accept hista-
mine 2, and only H3 receptors accept histamine 3.
The ANTIHISTAMINE MEDICATIONS to relieve allergy
symptoms have no effect on gastric acid produc-
tion. Similarly, the H2 blockers have no effect on
allergies.
Doctors may prescribe H2 blockers to treat GAS-
TROESOPHAGEAL REFLUX DISORDER(GERD), PEPTIC ULCER
DISEASE, chronic GASTRITIS, Crohn’s disease that
involves the stomach and ESOPHAGUS, HIATAL
HERNIA, and other conditions in which excessive
gastric acid causes symptoms or tissue damage.
The four H2 blockers used in the United States are
available in over-the-counter (OTC) and prescrip-
tion strengths.COMMON H2 BLOCKERS
H2 Blocker Prescription OTC Strength
Strength
cimetidine 200mg, 300mg, 100mg
(Tagamet) 400mg, 800mg
famotidine (Pepcid) 20mg, 40mg 10mg
nizatidine (Axid) 150mg, 300mg 75mg
ranitidine (Zantac) 150mg, 300mg 75mgH2 blockers, most notably cimetidine, interact
with numerous other medications. ANTACIDSpre-
vent the stomach from absorbing H2 blockers, sig-
nificantly reducing the effectiveness of the H2
blocker. Side effects of H2 blockers may include
dizziness, HEADACHE, and DIARRHEA. Changing to a
different H2 blocker medication often resolves any
side effects.
See also DIGESTIVE HORMONES; PROTON PUMP
INHIBITOR(PPI) MEDICATIONS.H2 antagonist (blocker) medications 59