Mild lopsidedness during speech is surprisingly normal, and most people do it without realizing. Research measuring lip opening in nearly 200 subjects found that about three-quarters of them opened the right side of their mouth wider when talking. But when asymmetry is pronounced enough that you or others notice it, the causes range from nerve problems and jaw conditions to simple muscle habits, and each one calls for a different fix.
Why Most People Already Talk a Little Lopsided
Before trying to correct anything, it helps to know that perfectly symmetrical speech does not exist. A study of 196 people found that 150 of them, roughly 76%, showed greater opening on the right side of the mouth during spontaneous speech. This held true across men and women, and across left-handers and right-handers alike.1PubMed. Mouth asymmetry during spontaneous speech The reason traces back to the brain. In most people, the left hemisphere runs expressive speech, and that same hemisphere has stronger motor control over the right side of the lower face. The result is a slight rightward bias in mouth movement that researchers can measure but listeners rarely detect.
The character of this asymmetry also shifts depending on what you are saying. When people produce purely verbal content, the right side of the mouth dominates. But during more emotionally loaded or imagery-heavy speech, the right hemisphere gets more involved, and the asymmetry can shift or flatten out.2International Journal of Psychology. Asymmetry in mouth opening during different speech tasks So if you notice lopsidedness only during certain kinds of talking, that is consistent with how the brain parcels out motor control. The question is whether your asymmetry falls within this normal range or has crossed into something that warrants attention.
Nerve Damage as a Cause
The most dramatic form of one-sided speech comes from facial nerve problems. Bell’s palsy is the best-known example: inflammation of the facial nerve, usually on one side, weakens or paralyzes the muscles of that half of the face. When this happens, the affected side droops, and talking, smiling, and even closing the eye become difficult. Research comparing people with unilateral facial palsy to healthy controls found that the palsy group had lower lip force, worse articulation, and more trouble controlling saliva.3PubMed. Peripheral facial palsy: Speech, communication and oral motor function If your one-sided speech appeared suddenly or alongside facial drooping, numbness, or difficulty closing one eye, a doctor should evaluate you right away. Bell’s palsy usually improves on its own within weeks to months, but early treatment with corticosteroids can speed recovery.
A less obvious nerve-related cause is synkinesis, which develops during recovery from facial nerve damage. When the nerve regenerates, it sometimes sends new fibers to the wrong muscles. The result is involuntary movement in one part of the face when you try to move another part. You might try to speak and find that one eye squints, or one cheek pulls in an unexpected direction. A systematic review of synkinesis research confirmed that the most widely accepted explanation is this misguided nerve regrowth after the original nerve fibers were damaged.4PubMed Central. Pathogenesis, diagnosis and therapy of facial synkinesis Synkinesis can make lopsided speech persistent and frustrating even after the initial palsy has healed, because the muscles are technically working again, just not in a coordinated way.
Jaw and Dental Problems That Pull Speech to One Side
Not all one-sided speech originates in the nerves. The jaw itself can drive asymmetry. Temporomandibular disorders, the cluster of conditions affecting the jaw joint and surrounding muscles, can change how your jaw moves during speech. A study comparing people with TMD to healthy controls found that TMD was associated with reduced jaw opening, and notably, a tendency toward one-sided jaw movements when talking.5PubMed. Mandibular movement patterns during speech in subjects with temporomandibular disorders and in asymptomatic individuals If the jaw joint on one side is stiff, inflamed, or clicking, the jaw naturally drifts toward or away from that side during speech, creating the appearance that you are talking out of one side.
The type of TMD matters. Research distinguishing between different forms found that people whose TMD involved the joint itself, rather than purely muscular pain, showed greater lateral jaw deviation during speech compared to controls.6CoDAS. Relation of painful TMD with the speech function If you also experience jaw clicking, popping, or pain when chewing, TMD is worth investigating as a root cause.
Dental alignment plays a role too. A systematic review found that unilateral posterior crossbite, where the upper teeth on one side sit inside the lower teeth, is associated with asymmetric muscle activity in the chewing muscles and lopsided jaw movement patterns.7European Journal of Orthodontics. Association between posterior crossbite, skeletal, and muscle asymmetry: a systematic review Because the same muscles that control chewing also shape mouth movements during speech, a crossbite can bias the jaw to one side whenever you talk. Orthodontic correction of the crossbite often improves this asymmetry over time.
Muscle Weakness and Long-Standing Habits
Sometimes the cause is neither nerve damage nor a structural jaw problem, but a pattern of muscle use that developed gradually. Chronic mouth breathing is one common culprit. When a child breathes through their mouth rather than their nose, often because of enlarged adenoids or allergies, the facial muscles develop differently. A study of children with mouth breathing found that over 80% had speech disorders, and the condition was linked to orofacial myofunctional problems, meaning the muscles of the lips, tongue, and cheeks were not working as they should.8PubMed Central. Mouth Breathing and Speech Disorders: A Multidisciplinary Evaluation Based on The Etiology These patterns can persist into adulthood, leaving some people with weak or poorly coordinated muscles on one side of the face.
Even without mouth breathing, habits accumulate. Consistently chewing food on one side, sleeping on one side, or holding tension in the jaw unevenly can gradually strengthen the muscles on one side more than the other. The brain’s motor system is remarkably good at adapting to whatever pattern you give it, which means bad habits get reinforced as readily as good ones. This is also why retraining is possible, because the same adaptability that let the pattern develop can be used to undo it.
How Feedback Mechanisms Keep Speech on Track, or Fail To
Understanding how the brain manages lip and jaw movement helps explain why retraining works. During speech, the brain does not simply send a fixed command to the facial muscles and hope for the best. Research on motor control during speech found that when the lower lip was unexpectedly perturbed during talking, corrective muscle responses appeared almost immediately, not just in the lip that was bumped but also in the upper lip, which is controlled by separate muscles.9PubMed. Control of complex motor gestures: orofacial muscle responses to load perturbations of lip during speech The system monitors itself in real time and makes adjustments on the fly. When one-sided speech becomes habitual, this feedback loop has essentially been trained to accept the asymmetry as normal. Retraining means giving the system new feedback, so it recalibrates.
Exercises and Biofeedback for Retraining
For cases driven by muscle weakness or habit rather than active nerve damage, targeted exercises are the first line of defense. Orofacial myofunctional therapy focuses on strengthening and coordinating the muscles of the lips, tongue, and cheeks. A study of women who performed structured orofacial exercises using an oral rehabilitation device for three minutes, four times a day, found significant improvements in lip closure strength and tongue elevation strength after 14 weeks of consistent practice.10Journal of Physical Therapy Science. Effect of Orofacial Myofunctional Exercise Using an Oral Rehabilitation Tool on Labial Closure Strength, Tongue Elevation Strength and Skin Elasticity The takeaway for anyone trying to correct one-sided speech is that facial muscles respond to consistent training the way any other muscles do, but you need to do the work daily for weeks before meaningful change shows up.
Some practical exercises you can do at home without specialized devices:
- Mirror practice: Sit in front of a mirror and slowly say vowel sounds, exaggerating your mouth opening. Watch for the side that lags and consciously try to match it to the more active side. Five to ten minutes twice a day is a reasonable starting point.
- Resistance exercises: Press a finger gently against the weaker side of your mouth and try to push outward with your lips against the resistance. Hold for five seconds, release, and repeat ten times.
- Symmetrical lip stretches: Smile as widely and evenly as possible, hold for five seconds, then pucker your lips tightly and hold for five seconds. Alternate for several minutes, checking in the mirror that both sides are moving equally.
- Reading aloud: Read a passage aloud while watching yourself in the mirror or recording a video. Focus on specific consonant sounds that feel most lopsided. Slow your speech rate to give the weaker side more time to engage.
For people recovering from facial nerve damage, biofeedback has shown real promise. The idea is to place sensors on the facial muscles and translate their electrical activity into a signal, like a tone or a visual readout, that the patient can use to learn how to activate weak muscles more effectively. An early case report demonstrated that a patient using surface electromyographic feedback during both speech and non-speech activities made substantial improvements in lip movements after nerve repair surgery.11PubMed. EMG feedback and recovery of facial and speech gestures following neural anastomosis More recently, a study of a child with congenital one-sided facial palsy found that real-time biofeedback roughly doubled the muscle activity on the affected side of the face over the course of just nine treatment sessions.12Biomedical Journal of Scientific & Technical Research. Effects of Real-Time iEMG Biofeedback on Facial Muscle Activation Patterns in a Child with Congenital Unilateral Facial Palsy Biofeedback typically requires a clinical setting and a trained therapist, but the results suggest it can accelerate what exercises alone might take much longer to accomplish.
Botulinum Toxin for Persistent Asymmetry
When one side of the face is overactive rather than weak, the problem is the opposite of what exercises fix. Botulinum toxin injections, commonly known by the brand name Botox, work by selectively weakening an overactive muscle so it matches the weaker side. A study of five patients with asymmetrical smiles caused by one-sided hyperactivity of a lip-depressing muscle found that injections leveled the lower lip and produced a symmetrical smile within one week. The effect lasted at least six months, and with repeated treatments, the duration gradually extended to an average of about seven months.13PubMed. Asymmetrical smiles corrected by botulinum toxin serotype A
Botulinum toxin has also been studied specifically for its effect on speech in people with synkinesis. Patients who received targeted injections showed statistically significant improvements in their ability to produce certain consonant sounds, particularly sounds like “s,” “z,” and “sh” that demand precise lip and tongue positioning. The improvements were accompanied by better speech-related quality of life scores.14PubMed. Effect of botulinum toxin injection on speech outcomes among patients with synkinetic facial nerve palsy This makes sense: if synkinesis causes unwanted muscle contractions that fight against the movements needed for speech, quieting those contractions clears the way for more symmetrical articulation.
The limitation is that botulinum toxin is temporary by nature. The injections need to be repeated every several months, and the dosing has to be carefully calibrated. Too much on one side and your speech swings lopsided in the other direction. This is a treatment best managed by a specialist, usually a neurologist or facial plastic surgeon who understands the anatomy of speech production.
Figuring Out Which Category You Fall Into
The right fix depends entirely on the cause, and the causes can overlap. A person might have both a jaw alignment issue and a muscle habit working together. A useful first step is recording yourself on video while reading a short passage aloud, then watching the playback at reduced speed. Pay attention to whether the asymmetry is in the lips, the jaw, or both. Lip-only asymmetry points toward nerve or muscle issues. Jaw deviation during speech suggests TMD or a dental problem. Clinicians can now measure lip and jaw range of motion with 3D motion-capture systems that track mouth corners and chin movement during specific vowel sounds, providing precise data on where the asymmetry is most pronounced.15PubMed. Range of motion (ROM) in the lips and jaw during vowels assessed with 3D motion analysis
For a thorough evaluation, the professionals to consider include a speech-language pathologist, a dentist or orthodontist if jaw or bite alignment seems relevant, and a neurologist if there is any suspicion of nerve involvement. Many speech-language pathologists have specific training in orofacial myofunctional therapy, which covers both the diagnostic assessment and the exercise-based treatment. If you have had a history of Bell’s palsy, facial surgery, or a stroke, that context is essential for whoever evaluates you, because the treatment approach for nerve-based asymmetry differs substantially from the approach for habitual or structural causes.
What to Expect from Retraining
Regardless of the approach, correcting one-sided speech takes patience. Facial muscles are small and fatigue quickly, and the neural pathways that control them are deeply ingrained. Most therapy protocols run a minimum of several weeks for habit-based asymmetry, and several months for nerve-recovery cases. The brain’s motor feedback system, the same one that corrects lip movements in real time, is ultimately what you are retraining. You are not just building muscle; you are teaching your brain to send different signals and accept a different pattern as its new default.
Progress tends to be uneven. You might see rapid improvement in certain sounds or movements and stubborn resistance in others. Sounds that require both sides of the mouth to work together symmetrically, like bilabial consonants (“b,” “p,” “m”), tend to improve faster because the brain has a clear target shape for the lips. Sounds that involve more complex configurations often take longer. Recording yourself periodically and comparing clips from different weeks is more reliable than your subjective impression, which tends to be colored by frustration on bad days and overconfidence on good ones.
People who combine multiple approaches tend to do best. Exercises build the muscular foundation, mirror or video feedback gives you real-time correction, and clinical biofeedback or botulinum toxin can fill in the gaps that self-directed practice cannot reach. The specific combination depends on the underlying cause, which is why getting an accurate diagnosis before committing to a treatment plan saves time in the long run.