Most muscle relaxers can be used while breastfeeding, though some are safer choices than others. The amount that passes into breast milk varies by medication, and a few carry enough risk that they’re best avoided entirely. The key is knowing which ones have safety data behind them and what to watch for in your baby.
How Drug Safety Is Measured During Breastfeeding
Researchers use something called the relative infant dose (RID) to estimate how much of a medication a baby actually receives through breast milk. It’s expressed as a percentage of the mother’s dose, adjusted for body weight. A RID below 10% is the long-standing rule of thumb for acceptability, though some experts now prefer a stricter 5% cutoff for extra margin. Drugs with a RID below 2% are considered minimal exposure.
For muscle relaxers specifically, the data is limited. Most of what we know comes from small case reports rather than large clinical trials, which is typical for breastfeeding research. But the available evidence does point to some clear patterns.
Cyclobenzaprine: The Most Studied Option
Cyclobenzaprine (the active ingredient in Flexeril) has the best data of any muscle relaxer during breastfeeding. In two nursing mothers, one taking 5 mg daily and the other taking 10 mg twice daily, the amount reaching breast milk was very small. The weight-adjusted infant dose came out to just 0.5% of the mother’s dose in both cases, well below the 10% safety threshold and even under the stricter 2% “minimal exposure” cutoff.
Neither infant showed any signs of sedation or other side effects. The LactMed database, maintained by the National Institutes of Health, states that if a mother needs cyclobenzaprine, it is not a reason to stop breastfeeding. One serious case was reported to U.S. poison control centers between 2001 and 2017: a 16-day-old infant exposed to cyclobenzaprine along with acetaminophen and oxycodone through breast milk developed breathing problems and a slow heart rate. That infant survived, but the combination of multiple sedating drugs in a very young newborn likely played a role.
Baclofen: Low Levels in Milk
Baclofen, often prescribed for muscle spasticity, also transfers into breast milk at low levels. In one mother taking 20 mg four times daily, peak milk concentrations averaged 0.343 mg/L. When baclofen is delivered through a spinal pump (intrathecal administration), the amount in milk drops dramatically. One mother receiving baclofen this way had a milk level of just 0.000617 mg/L, essentially negligible.
LactMed recommends monitoring newborns for sedation but does not advise against breastfeeding while taking baclofen.
Tizanidine and Methocarbamol: No Human Data
Tizanidine (Zanaflex) and methocarbamol (Robaxin) are both commonly prescribed muscle relaxers, but neither has published data on how much reaches breast milk or how infants respond to the exposure. No studies have measured milk levels, and no infant outcomes have been formally reported for either drug.
Despite this gap, LactMed does not consider either medication a reason to stop breastfeeding if the mother needs it. The recommendation for both is to watch the baby for drowsiness, poor feeding, and adequate weight gain. That said, the lack of data means there’s more uncertainty compared to cyclobenzaprine or baclofen, and your prescriber may prefer one of the better-studied options.
Carisoprodol: The One to Avoid
Carisoprodol (Soma) stands apart from other muscle relaxers because your body breaks it down into meprobamate, a substance with sedative properties that also carries dependence risk. Meprobamate’s sedating effects and its potential to accumulate make carisoprodol a poor choice during breastfeeding. If you’re currently taking it and breastfeeding, it’s worth discussing a switch with your provider.
What to Watch For in Your Baby
All muscle relaxers are sedating to some degree, so the signs to monitor are related to your baby’s alertness and feeding behavior. Specifically, look for:
- Unusual sleepiness or difficulty waking for feedings
- Poor latching or feeding, including weak sucking
- Limpness or reduced responsiveness to stimulation
- Breathing changes, including shallow breathing or any bluish tint around the lips
- Slow weight gain over days to weeks
These concerns are highest in newborns under four weeks old and in premature infants, whose livers are less efficient at processing medications. The risk also increases when you’re taking more than one sedating medication at the same time. If you notice any of these signs, contact your pediatrician promptly.
Timing Your Dose to Reduce Exposure
A simple strategy to minimize how much medication your baby gets is to take your dose right after nursing or at the start of a feeding session. Most drugs reach their peak concentration in breast milk within one to four hours. If your next feeding falls two to three hours later, levels will typically be dropping by then. This won’t eliminate exposure entirely, but it can meaningfully reduce the peak amount your baby receives.
For cyclobenzaprine specifically, the peak milk concentration occurred within the first few hours after a dose, so nursing just before taking the medication gives you the widest window before the next feeding.
Alternatives Worth Trying First
Depending on what’s causing your muscle pain, you may not need a muscle relaxer at all. Ibuprofen is the preferred first-line pain reliever during breastfeeding because very little passes into milk, and its short half-life of about 1.8 hours means levels drop quickly between feedings. Acetaminophen is also safe during breastfeeding. Both are available over the counter and can be effective for muscle pain, especially when combined with non-drug approaches.
For back, neck, or joint pain (common postpartum from the physical strain of pregnancy and delivery), a heating pad, gentle stretching, and physical therapy often provide substantial relief. Postpartum back pain frequently stems from weakened abdominal muscles and hormonal changes that loosen joints, so targeted strengthening exercises can address the root cause rather than just managing symptoms.
If over-the-counter options aren’t enough, your provider can help weigh whether a muscle relaxer with better safety data, like cyclobenzaprine at the lowest effective dose, makes sense for your situation.