Mechanism of action
Bisacodyl (Bisacodylum) belongs to the group of contact laxatives. Chemically, it is a synthetic drug, a diphenylmethane derivative.
Its mechanism of action is resorptive. After oral administration, the drug is absorbed in the small intestine and then secreted back into the lumen of the large intestine. In addition, the drug is cleaved in the alkaline environment of the intestine. As a result, active metabolites are formed that cause direct irritation of the mucous membrane chemoreceptors. This leads to a reflex increase in peristalsis.
It is important to emphasize that bisacodyl acts exclusively in the large intestine. This fundamentally distinguishes it from saline laxatives (such as magnesium sulfate or sodium sulfate), which act throughout the entire gastrointestinal tract. It also differs from osmotic agents and bulk-forming agents (kelp, methylcellulose), as it does not work by increasing volume or lubrication (unlike oils).
Pharmacological effects
The main pharmacological effect of the drug is laxative. The strength of this effect is rated as moderate (less pronounced than that of saline laxatives).
A key pharmacokinetic feature of bisacodyl is the strict dependence of the latency period (the time from administration to the onset of effect) on the route of administration:
- For oral administration (per os), the effect occurs in 5–7 hours.
- For rectal administration (as suppositories), the action develops significantly faster — within 1 hour.
Position in classification and comparison with analogues
In the classification of laxatives, bisacodyl is classified as a synthetic drug that irritates chemoreceptors. This same subgroup includes phenolphthalein, oxyphenisatine, and sodium picosulfate.
- Phenolphthalein is considered the reference agent of the group; however, with prolonged use, it carries a risk of accumulation and can exert an irritating effect on the kidneys.
- Oxyphenisatine is an analogue of phenolphthalein, but less toxic.
- Sodium picosulfate, unlike bisacodyl, is not absorbed in the small intestine at all and is hydrolyzed by the bacterial flora of the large intestine with the release of free active diphenol.
Also, bisacodyl should be distinguished from herbal laxatives containing anthraglycosides and oils (Senade, palmate rhubarb root, glossy buckthorn bark, castor oil), and from disaccharides (lactulose).
Indications for use
In clinical practice, the drug is prescribed for the following tasks:
- Treatment of constipation of various etiologies (to facilitate bowel evacuation).
- Preoperative preparation (for adequate bowel cleansing before surgical interventions).
- Food poisoning (as part of complex therapy for the accelerated removal of toxins from the gastrointestinal tract).
Prescribing specifics and formulation
The drug is available in two dosage forms: 0.005 g tablets and 0.01–0.02 g rectal suppositories.
Oral administration: prescribed internally 30 minutes before breakfast. Tablets must be swallowed whole without chewing so that the drug reaches the alkaline environment of the intestine. A single dose is 1–2 tablets. The maximum single dose (MSD) is 0.025 g, the maximum daily dose (MDD) is 0.05 g.
Rectal administration: prescribed 1–2 suppositories per day rectally.
Example of tablet prescription: ```latin Rp.: Tab. Bisacodyli 0,005 D.t.d. N. 30 S. Take orally 1-2 tablets 30 minutes before breakfast, without chewing. ```
Example of suppository prescription: ```latin Rp.: Supp. cum Bisacodylo 0,01 D.t.d. N. 10 S. Rectally 1 suppository 1 time per day. ```
Briefly: answers to topical questions
What side effects does bisacodyl cause?
Bisacodyl causes a number of adverse reactions, the risk of which increases significantly with prolonged use of high doses. The main side effects of the drug include:
- Diarrhea accompanied by flatulence and cramp-like abdominal pain.
- Fluid loss and electrolyte disturbances (in particular, hypokalemia).
- Degenerative changes in the cells of Meissner's and Auerbach's plexuses.
- Habituation effect, leading to the development of lazy bowel syndrome.
Bisacodyl hits fast in suppositories (effect up to 1 hour), while in tablets it waits for the morning (latency period 5–7 hours).
Class and exam questions
How does bisacodyl differ from magnesium sulfate in the localization of its action?
Bisacodyl acts exclusively in the large intestine, irritating its chemoreceptors. Magnesium sulfate (a saline laxative) acts throughout the entire intestine.
Why must bisacodyl tablets be taken without chewing?
The drug must reach the alkaline environment of the intestine for proper cleavage and activation. Chewing will disrupt this process.
How quickly will the effect occur with different routes of administration?
When taken orally, the latency period is 5–7 hours. When administered rectally (suppositories), the effect occurs within 1 hour.
What else to study on this topic
- Comparative characteristics of synthetic laxatives (phenolphthalein, sodium picosulfate).
- Mechanism of action of saline laxatives (magnesium sulfate, sodium sulfate).
- Herbal laxatives (Senade, rhubarb root, castor oil).
- Agents increasing intestinal content volume (kelp, methylcellulose).
- Principles of pharmacotherapy for food poisoning and bowel cleansing.
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