Muscle relaxers cause drowsiness in up to 40% of people who take them, making it the single most common side effect. Beyond sedation, these medications can trigger dry mouth, dizziness, nausea, constipation, and fatigue. Because they work by slowing activity in the brain and nervous system, most of their side effects stem from that same calming action spreading beyond the muscles you’re trying to treat.
The Most Common Side Effects
The side effects you’re most likely to notice are the ones tied to how muscle relaxers work. They dampen signals in your brain and spinal cord, which relaxes tight muscles but also slows down other processes. The drowsiness tends to kick in fast, often within 30 minutes to an hour of taking a dose, and can last four to six hours depending on the medication.
The everyday side effects most people experience include:
- Drowsiness and fatigue: The most frequent complaint, affecting a significant number of users and strong enough to impair driving
- Dizziness or lightheadedness: Especially when standing up quickly
- Dry mouth: A result of the drug’s effects on nerve signaling throughout the body
- Nausea and heartburn
- Constipation: Because the same nerve-calming effect slows digestion
- Blurred vision
These side effects are usually mild and tend to ease as your body adjusts over the first few days. That said, the sedation can be significant enough to affect your ability to work, drive, or think clearly, especially during the first week.
How Side Effects Differ by Type
Not all muscle relaxers are the same. There are two broad categories, and they produce somewhat different side effect profiles. The ones prescribed for back pain, neck spasms, and similar musculoskeletal injuries act on your skeletal muscles by working through the brain. These tend to cause the classic sedation, dry mouth, and dizziness.
The other category targets smooth muscles inside your organs, the kind involved in conditions like irritable bowel syndrome or bladder spasms. These can cause dry eyes, urinary retention (difficulty emptying your bladder), fast heart rate, and headaches in addition to dizziness and dry mouth. Some people with digestive conditions use peppermint oil as a milder option in this category, though it can still cause heartburn and nausea.
Among the skeletal muscle relaxers, the level of sedation varies. Some produce stronger calming and even euphoric effects, which is part of why certain ones carry a higher risk of misuse. Others are milder but still cause noticeable drowsiness.
Serious Reactions to Watch For
Rare but serious side effects do occur. A condition called serotonin syndrome can develop when muscle relaxers are combined with other medications that raise serotonin levels in the brain, such as certain antidepressants. Symptoms include agitation, fever, sweating, confusion, fast or irregular heartbeat, severe muscle stiffness or twitching, hallucinations, and loss of coordination. This is a medical emergency.
Other serious reactions include severe allergic responses with facial swelling, hives, or difficulty breathing. Some people experience chest pain, irregular heart rhythms, or significant drops in blood pressure. A large-scale analysis of reports to the FDA also flagged confusion, delirium, and falls as notable adverse events, particularly concerning for anyone already at risk for these problems.
The Risk of Dependency
Muscle relaxers can be addictive, particularly when taken at higher doses or for longer than a few weeks. Some types carry more risk than others. The ones that activate receptors in the brain controlling relaxation and anxiety can produce a sense of calm or mild euphoria that becomes psychologically rewarding. Over time, the brain adjusts, requiring higher doses for the same relief.
When someone stops taking muscle relaxers after prolonged use, withdrawal symptoms typically begin within 12 to 24 hours. The first few days often bring insomnia, irritability, headaches, and restlessness. These symptoms tend to peak around day three and gradually improve after a week, though emotional symptoms like low mood or cravings can linger for several weeks. Tapering off slowly rather than stopping abruptly makes a significant difference.
One medication used for spasticity carries a particularly notable withdrawal risk. Stopping it suddenly, especially at high doses, can trigger severe symptoms including seizures, hallucinations, high fever, and dangerous changes in heart rate and blood pressure. In at least one documented case, abrupt withdrawal mimicked a life-threatening neurological emergency. This is why doctors taper the dose gradually when discontinuing it.
Why Alcohol Makes Everything Worse
Mixing muscle relaxers with alcohol is one of the most dangerous combinations because both substances slow down the same systems in your body. Alcohol amplifies the sedative effects, which can suppress your breathing rate, drop your blood pressure, and severely impair coordination and judgment. The risk of overdose climbs sharply.
Specific dangers of combining the two include extremely slowed breathing, confusion, seizures, heart rhythm abnormalities, loss of motor control, and memory problems. Even one or two drinks can push the sedative effects into a dangerous range, especially during the first few hours after taking a dose when the drug’s effects are strongest.
Higher Risks for Older Adults
If you’re over 65, muscle relaxers carry extra concern. The American Geriatrics Society places them on its Beers Criteria, a widely used list of medications considered potentially inappropriate for older adults because the risks outweigh the benefits. The specific concerns are confusion, dry mouth, and constipation, all of which tend to be more pronounced and more consequential in older bodies.
Older adults metabolize these drugs more slowly, so the sedation lasts longer and hits harder. The resulting drowsiness and dizziness significantly increase fall risk, which is especially dangerous when bone density is already declining. Confusion caused by muscle relaxers can also be mistaken for cognitive decline, leading to unnecessary worry or misdiagnosis.
How Long Side Effects Typically Last
For most muscle relaxers, the sedative effects last four to six hours per dose. It can take up to two days to feel the full muscle-relaxing benefit, and up to a week for maximum relief from spasm pain. During that ramp-up period, side effects like drowsiness and dizziness are often at their most noticeable as your body adjusts.
Most people are prescribed muscle relaxers for short periods, typically two to three weeks. When used within that window and at the prescribed dose, side effects generally stay manageable. The problems tend to escalate with longer use, higher doses, or combinations with alcohol and other sedating substances.