Do muscle relaxers cause constipation?

In this article:

• Muscle relaxer constipation
• Why do muscle relaxers cause constipation?
• Muscle relaxer side effects
• Preventing constipation
• When to see a doctor

In this article

• Muscle relaxer constipation
• Why do muscle relaxers cause constipation?
• Muscle relaxer side effects
• Preventing constipation
• When to see a doctor

In a perfect world, over-the-counter and prescription medications and supplements would treat symptoms without causing any side effects. But in reality, most come with at least a few unwanted ones. Common culprits include skeletal muscle relaxants, which are commonly prescribed for muscle spasms or back pain, and central nervous system (CNS) depressants like benzodiazepines,  which are typically used for anxiety but also have muscle-relaxing effects.

Muscle relaxers can also slow digestion. Constipation isn’t the most common side effect, but it does occur. Here’s why it happens and what you can do to keep things moving.

Can muscle relaxers cause constipation?

“Constipation is a side effect, but it is not a common side effect of taking muscle relaxants,” says Sandhya Shukla, MD, a board-certified gastroenterologist with Atlantic Coast Gastroenterology Associates in New Jersey. “In the safety data, constipation is usually reported in 1% to 5% of patients using commonly prescribed muscle relaxants such as cyclobenzaprine, tizanidine, methocarbamol, or baclofen.” Dr. Shukla adds that there are other, more common gastrointestinal side effects of taking muscle relaxers, including nausea and indigestion.

Why do muscle relaxers cause constipation?

There are two types of muscle relaxers: antispasmodic medications (which are mostly used to treat muscle spasms and pain) and antispastic medications (which are mostly used to treat spasticity). These two classes of muscle relaxers work in slightly different ways but can both cause constipation. “Antispasmodics relax the smooth muscle of the intestines and antispastics affect nerve signals and muscle tone, so either can reduce bowel movement activity,” explains internist and gastroenterologist Supriya Rao, MD. 

However, Dr. Shukla says that constipation is much more frequently seen as a side effect of antispasmodics. “Antispasmodics cause constipation primarily through their anticholinergic effects. Essentially, they reduce the movement of stool through your colon and slow transit in the GI system as a whole,” she explains. Anticholinergic refers to the fact that the muscle relaxers block the neurotransmitter acetylcholine from telling the muscles of the digestive system to contract and move food along, resulting in decreased motility. “When the gut muscles are more relaxed than they should be, the stool sits longer in the colon, more water is absorbed, and bowel movements become harder and less frequent,” Dr. Rao says.

Antispasmodic muscle relaxers include:

• Soma (carisoprodol)
• Lorzon (chlorzoxazone)
• Flexeril (cyclobenzaprine)
• Metaxall (metaxalone)
• Robaxin (methocarbamol)
• Norflex (orphenadrine) 

Antispastic muscle relaxers include: 

• Ozobax (baclofen)
• Dantrium (dantrolene)

There are also two medications that have both antispasmodic and antispastic effects: 

• Zanaflex (tizanidine) 
• Valium (diazepam)

What are the most common side effects of muscle relaxers?

In addition to constipation, there are other potential side effects you could experience from taking muscle relaxers. They vary slightly based on the medication type.

Antispasmodic muscle relaxer side effects

For example, cyclobenzaprine (a commonly prescribed antispasmodic muscle relaxer) can also cause:

• Dizziness
• Drowsiness and fatigue
• Dry mouth
• Nausea

While the above side effects usually don’t require medical attention, you should call your healthcare provider if you experience any of the following side effects:

• Signs of an allergic reaction, such as hives, skin rash, itching, and tongue or facial swelling
• Slow or shallow breathing or shortness of breath
• Confusion
• Trouble staying awake
• Feeling faint or lightheaded
• Fast or irregular heartbeat
• Chest pain

Antispastic muscle relaxer side effects

Meanwhile, the most common side effects of baclofen, an antispastic muscle relaxer, include:

• Confusion
• Dizziness or lightheadedness
• Headache
• Increased need to urinate
• Nausea
• Sweating
• Trouble sleeping
• Unusual tiredness or weakness

Serious side effects are less common, but they can occur. If you experience any of the following, reach out to your healthcare provider for medical advice:

• Bloody or dark urine
• Chest pain
• Issues with muscle control or unexplained stiffness
• Fainting
• Mood changes
• Pounding heartbeat
• Ringing or buzzing in the ears
• Seeing or hearing things that aren’t there
• Skin rash
• Slurred speech
• Ankle swelling
• Unusual excitement

In addition to these standard side effects, it’s important to know that there are many other medications that could potentially interact with muscle relaxers. Make sure to talk to a healthcare professional and your pharmacist about possible drug interactions with any OTC, prescription drugs, or supplements you’re taking before you start a muscle relaxer.

How to prevent constipation from muscle relaxers

Drs. Rao and Shukla say that their first-line recommendations for preventing constipation from muscle relaxers are to increase your intake of fiber and water. Healthy high-fiber foods include nut butters, beans, lentils, whole-wheat bread, apples, berries, broccoli, Brussels sprouts, leafy greens, and potatoes. You should also aim to drink at least 68 ounces of water per day. Together, fiber and water will make your stool bigger and softer so your GI tract can push it out.

“If you experience constipation while using a muscle relaxer, and dietary measures are insufficient, osmotic laxatives such as polyethylene glycol (PEG) or magnesium salts are recommended as first-line pharmacologic therapy,” Dr. Shukla says. Osmotic laxatives pull water into your colon, softening your stool. If neither of these approaches works, Dr. Shukla says you could try a stimulant laxative. These activate the nerves that prompt the muscles in the colon to work harder to move your stool along. 

When to see a healthcare provider

While it can happen occasionally, constipation is not one of the most common side effects of muscle relaxers. Chances are, even if you do experience constipation, it won’t be for long. “Muscle relaxers are intended for short-term use, so the constipation should go away when you stop taking them,” Dr. Rao says. If it doesn’t and the laxatives mentioned above don’t work either, it’s worth reaching out to your healthcare provider. You should also get evaluated if you experience any “red flag” symptoms that could indicate an underlying problem, Dr. Shukla says. These include rectal bleeding, blood mixed in the stool, unexplained weight loss, or a sudden or significant change in bowel habits.