Bethanechol chloride acts principally by producing the
effects of stimulation of the parasympathetic nervous system. It increases the
tone of the detrusor urinae muscle, usually producing a contraction sufficiently
strong to initiate micturition and empty the bladder. It stimulates gastric
motility, increases gastric tone and often restores impaired rhythmic
Stimulation of the parasympathetic nervous system
releases acetylcholine at the nerve endings. When spontaneous stimulation is
reduced and therapeutic intervention is required, acetylcholine can be given, but
it is rapidly hydrolyzed by cholinesterase and its effects are transient.
Bethanechol chloride is not destroyed by cholinesterase and its effects are
more prolonged than those of acetylcholine.
Effects on the GI and urinary tracts sometimes appear
within 30 minutes after oral administration of bethanechol chloride, but more
often 60 to 90 minutes are required to reach maximum effectiveness. Following
oral administration, the usual duration of action of bethanechol is one hour,
although large doses (300 to 400 mg) have been reported to produce effects for
up to six hours. Subcutaneous injection produces a more intense action on
bladder muscle than does oral administration of the drug.
Because of the selective action of bethanechol, nicotinic
symptoms of cholinergic stimulation are usually absent or minimal when orally
or subcutaneously administered in therapeutic doses, while muscarinic effects
are prominent. Muscarinic effects usually occur within 5 to 15 minutes after subcutaneous
injection, reach a maximum in 15 to 30 minutes, and disappear within two hours.
Doses that stimulate micturition and defecation and increase peristalsis do not
ordinarily stimulate ganglia or voluntary muscles. Therapeutic test doses in
normal human subjects have little effect on heart rate, blood pressure or
Bethanechol chloride does not cross the blood-brain
barrier because of its charged quaternary amine moiety. The metabolic rate and
mode of excretion of the drug have not been elucidated.
A clinical study (Diokno, AC.; Lapides, J.; Urol 10 :
23-24, July 1977) was conducted on the relative effectiveness of oral and
subcutaneous doses of bethanechol chloride on the stretch response of bladder
muscle in patients with urinary retention. Results showed that 5 mg of the drug
given subcutaneously stimulated a response that was more rapid in onset and of
larger magnitude than an oral dose of 50 mg, 100 mg, or 200 mg. All the oral
doses, however, had a longer duration of effect than the subcutaneous dose.
Although the 50 mg oral dose caused little change in intravesical pressure in
this study, this dose has been found in other studies to be clinically
effective in the rehabilitation of patients with decompensated bladders.