Flexeril (cyclobenzaprine) is not considered harmful to the kidneys at normal doses. Unlike common pain relievers such as ibuprofen or naproxen, cyclobenzaprine is not known to directly damage kidney tissue or reduce kidney function. The drug is processed primarily by the liver, not the kidneys, which is why the FDA labeling includes dose adjustments for liver impairment but not for kidney impairment. That said, there are a few indirect ways Flexeril can stress the urinary system, and overdose is a different story entirely.
How Flexeril Is Processed in Your Body
Cyclobenzaprine is broken down extensively by the liver before it leaves your body. This is a key distinction from medications like NSAIDs, which are filtered through the kidneys and can reduce blood flow to them over time. Because the liver does the heavy lifting with Flexeril, the drug does not place a significant filtering burden on the kidneys under normal circumstances.
The FDA’s prescribing information for Flexeril includes specific warnings and dose reductions for people with liver problems but does not list kidney disease as a reason to adjust the dose or avoid the drug. Similarly, the 2023 American Geriatrics Society Beers Criteria, which flags medications that are risky for older adults with reduced kidney function, does not include cyclobenzaprine on its kidney-specific caution list.
Urinary Side Effects to Watch For
While Flexeril doesn’t damage the kidneys directly, it does have anticholinergic properties, meaning it blocks a chemical messenger involved in muscle contractions throughout the body, including the bladder. This can lead to urinary retention, where the bladder doesn’t empty completely. In a large pharmacovigilance analysis published in Frontiers in Medicine that reviewed 2,425 adverse event reports linked to cyclobenzaprine, 108 involved renal and urinary disorders. Urinary retention appeared almost exclusively in male patients, likely because an enlarged prostate compounds the problem.
Urinary retention isn’t kidney damage in itself, but if urine backs up significantly or goes unrecognized for a prolonged period, it can increase pressure on the kidneys and raise the risk of urinary tract infections. If you notice difficulty starting urination, a weak stream, or a feeling that your bladder isn’t fully emptying while taking Flexeril, that’s worth bringing up with your prescriber.
The Overdose Exception
At therapeutic doses, Flexeril poses minimal kidney risk. Overdose is a completely different scenario. A case report published in the journal CHEST documented a patient who overdosed on cyclobenzaprine and developed rhabdomyolysis, a condition where damaged muscle fibers release their contents into the bloodstream. Those byproducts can clog the kidneys’ filtering units, leading to acute kidney injury. The patient’s creatinine level, a key marker of kidney function, spiked to 7.1 mg/dL (normal is roughly 0.6 to 1.2). She also experienced liver failure, shock, and respiratory failure. The authors noted this was the first reported case of life-threatening multiorgan failure from cyclobenzaprine overdose. Importantly, her kidney function returned to normal within days once she received supportive care.
This type of kidney injury is not unique to Flexeril. Any drug that causes rhabdomyolysis or severe toxicity can harm the kidneys as a downstream consequence. It reflects the danger of overdose rather than a property of the medication at prescribed doses.
Taking Flexeril With Pain Relievers
People prescribed Flexeril for muscle spasms often take it alongside over-the-counter pain relievers like ibuprofen (Advil) or naproxen (Aleve). Those NSAIDs are the ones that carry real kidney risk, especially with long-term use, dehydration, or preexisting kidney problems. The FDA label notes that combining Flexeril with naproxen or diflunisal was “well tolerated with no reported unexpected adverse effects,” though the combination did cause more drowsiness than the pain reliever alone. There is no documented evidence that Flexeril amplifies the kidney risks of NSAIDs.
Still, if you’re taking both medications and you have any degree of kidney impairment, the NSAID is the ingredient to be more cautious about, not the Flexeril.
Concerns for Older Adults
The Beers Criteria, a widely used guide for medication safety in people 65 and older, recommends avoiding Flexeril in this age group. The concern isn’t the kidneys, though. It’s the drug’s anticholinergic side effects: excessive drowsiness, confusion, dry mouth, constipation, and an increased risk of falls and fractures. These effects are harder for older bodies to tolerate, and the evidence that Flexeril works well at doses older adults can handle is weak.
Older adults do tend to have naturally lower kidney function, which can slow the clearance of many drugs. While Flexeril is liver-metabolized, age-related changes in both liver and kidney function can allow the drug to linger longer in the body, intensifying side effects. This is another reason the Beers Criteria flags it as potentially inappropriate for this population, even though the kidneys themselves aren’t the target of harm.
Who Should Be More Cautious
If you have healthy kidneys and take Flexeril as prescribed for short-term muscle spasm relief (typically two to three weeks), kidney damage is not a realistic concern. The people who should pay closer attention include those already living with chronic kidney disease, older adults with declining organ function, men with prostate enlargement who may be more susceptible to urinary retention, and anyone taking multiple medications that affect the kidneys.
Flexeril is not in the same category as NSAIDs, certain antibiotics, or contrast dyes when it comes to kidney risk. For most people, the side effects worth watching for are the more common ones: drowsiness, dry mouth, and dizziness.