Muscle relaxers are prescription medications used to relieve pain, stiffness, and involuntary muscle spasms by acting on the central nervous system. A frequently reported side effect is an alteration in normal digestive function, leading to constipation. This disruption occurs because the mechanisms that relax skeletal muscles can unintentionally affect the smooth muscle responsible for moving food through the digestive tract. Understanding this connection and knowing how to manage the symptom is important for anyone starting these medications.
How Muscle Relaxers Affect Digestion
Muscle relaxers calm the central nervous system, reducing the nerve signals that cause muscles to contract. This dampening effect extends to the smooth muscles lining the gastrointestinal (GI) tract. The movement of food and waste through the intestines, known as peristalsis, relies on coordinated contractions of these smooth muscles. When the central nervous system slows down, the frequency and strength of intestinal contractions decrease, leading to sluggish gut motility.
Many muscle relaxers possess anticholinergic properties, blocking the action of the neurotransmitter acetylcholine. Acetylcholine stimulates muscle contraction, including the involuntary movements of the gut that propel contents forward. By inhibiting acetylcholine, these medications reduce the signal for intestinal muscles to contract, causing waste to move slowly through the colon. This prolonged transit time allows the colon to absorb excessive water from the stool, resulting in hard, dry, and difficult-to-pass bowel movements, which defines constipation.
Muscle Relaxer Agents Linked to Constipation
The risk of constipation varies among different muscle relaxer agents, depending on their chemical structure and the strength of their anticholinergic effects. Cyclobenzaprine (Flexeril) is a frequently cited culprit due to its strong anticholinergic properties. Constipation is commonly reported in patients taking cyclobenzaprine, often in the range of 1% to 5% during clinical trials. Its mechanism of action and effect on the acetylcholine system make it a high-risk agent for digestive slowing.
Other agents, such as baclofen, used to treat spasticity, and carisoprodol (Soma), also carry a risk of causing constipation. Baclofen has a moderate risk level, acting as a central nervous system depressant that slows gut motility. Carisoprodol is in the moderate to high-risk category because its sedative effects can reduce physical activity and contribute to sluggish digestion. Medications like tizanidine (Zanaflex) and methocarbamol (Robaxin) are also associated with constipation, though they pose a lower to moderate risk compared to cyclobenzaprine.
Managing Constipation While on Medication
Addressing medication-induced constipation begins with simple changes to diet and lifestyle. Increasing dietary fiber intake is a primary recommendation, as fiber adds bulk to the stool, helping it retain water and move easily through the intestines. Sources like fruits, vegetables, whole grains, and legumes should be incorporated gradually to prevent excessive gas and bloating. Adequate hydration is equally important, as water softens the stool and makes the fiber more effective in promoting regular bowel movements.
Physical activity also helps stimulate the bowels; even light exercise, such as a daily walk, encourages intestinal muscle contractions and improves gut function. If dietary and lifestyle adjustments are not sufficient, over-the-counter (OTC) options can provide relief.
Over-the-Counter Options
Osmotic laxatives, such as polyethylene glycol (MiraLAX), are often recommended first, as they work by drawing water into the colon to soften the stool. Stool softeners, like docusate sodium, are also helpful by mixing water and fat into the stool, making it easier to pass. Bulk-forming laxatives, which contain fiber supplements like psyllium, can be used but must be taken with plenty of water to avoid obstruction. Stimulant laxatives are reserved for when other treatments fail, as they directly cause the muscles in the colon to contract. It is important to consult with a healthcare provider or pharmacist before starting any new laxative regimen to ensure it is appropriate for the specific medication and individual health status.
Recognizing Severe Symptoms
While most medication-related constipation is manageable, certain symptoms indicate a serious condition requiring immediate medical consultation. Severe abdominal pain that is intense and constant, or pain that becomes debilitating, should not be managed at home. This discomfort can signal complications such as fecal impaction or, rarely, a bowel obstruction.
The inability to pass gas or stool for a prolonged period, typically seven days or more, warrants medical attention, as this signals a possible blockage. Other severe warning signs include persistent nausea and vomiting, especially if vomiting occurs without relief. The presence of blood in the stool, whether bright red or dark and tarry, must be evaluated by a medical professional immediately. These symptoms suggest an issue beyond simple constipation and may require urgent intervention.