Drooping mouth corners result from the combined effects of a specific facial muscle, age-related tissue changes, and sometimes neurological or dental conditions. The depressor anguli oris, a thin fan-shaped muscle just below each corner of the mouth, is the primary structure responsible for pulling the mouth corners downward. When this muscle becomes overactive, or when the tissues that normally counterbalance it weaken and thin with age, the corners of the mouth settle into a downward slope that can make a person look sad, tired, or stern even at rest.
The Muscle That Pulls Your Mouth Down
Your mouth corners sit at the intersection of several muscles that pull in competing directions. Some lift the corners when you smile, while others depress them. The depressor anguli oris (DAO) is the main downward-pulling muscle, originating along the lower jawbone and inserting into the tissue at the corner of the mouth called the modiolus. When you frown or express displeasure, the DAO contracts and drags the corners down.1PubMed Central. Novel anatomical proposal for botulinum neurotoxin injection targeting depressor anguli oris for treating drooping mouth corner In youth, the upward-pulling muscles (the zygomaticus major and minor, and the levator muscles) keep things in balance at rest, so the corners stay roughly horizontal or slightly lifted. But when the DAO becomes hyperactive or the opposing muscles weaken, the balance tips. The result is a persistent downturn that shows up even when you are not making any particular expression.2Plastic & Reconstructive Surgery. Anatomical Considerations Regarding the Location and Boundary of the Depressor Anguli Oris Muscle with Reference to Botulinum Toxin Injection
This muscular tug-of-war explains why some people notice their mouth corners drooping on one side more than the other. Slight asymmetries in DAO bulk or activity are normal, but when one side dominates, it creates a lopsided appearance that can be more noticeable than a symmetrical downturn.
How Aging Reshapes the Lower Face
Aging is by far the most common reason otherwise healthy people develop drooping mouth corners. The process is not a single event but a slow cascade that involves nearly every layer of tissue in the lower face.
It starts deeper than most people realize. The facial skeleton itself changes shape with age. The maxilla (upper jaw) and the mandible (lower jaw) both lose bone over time, particularly in a zone called the prejowl area of the mandible, just below and to the sides of the chin. As this bony platform shrinks, the soft tissues it was supporting lose their scaffolding and sag.3PubMed Central. Changes in the Facial Skeleton With Aging: Implications and Clinical Applications in Facial Rejuvenation Bone resorption in the midface, including around the nose and the orbital rims, also contributes by allowing the cheek tissues to slide downward, adding bulk and weight to the lower face that wasn’t there before.
On top of the bone, fat compartments in the face shift and shrink unevenly. The cheek fat pads that once provided volume high on the face deflate and descend, while the fat pockets near the jawline can actually enlarge. This redistribution amplifies the visual impression of heaviness around the mouth. Ligaments that once held fat and skin tightly against the bone stretch and loosen, and the skin itself thins and loses elasticity as collagen breaks down. Gravity does the rest. All of these forces converge at the mouth corners, producing the creases that run from the corners of the mouth down toward the jawline, commonly known as marionette lines.4PubMed Central. Why do marionette lines appear? Exploring the anatomical perspectives and role of thread-based interventions
People often assume drooping mouth corners are purely a skin problem, something that a good moisturizer might prevent. In reality, the process is multi-layered. Bone loss, fat redistribution, ligament laxity, muscle changes, and skin aging all contribute, and addressing only one layer usually produces a limited result.
When Nerve Damage Is the Cause
A sudden or noticeable droop on one side of the mouth raises a very different set of concerns than the gradual, symmetrical downturn of aging. Facial nerve injury is one of the most important causes of acute, asymmetric mouth drooping. Bell’s palsy, which involves inflammation or compression of the facial nerve on one side, is the most common form. It typically strikes without warning, weakening or paralyzing the muscles on one half of the face, including the muscles that lift the mouth corner. Stroke can produce a similar appearance, though the pattern of weakness differs depending on where in the brain the damage occurs.
Even after the acute phase of Bell’s palsy resolves, some people develop a complication called synkinesis, where nerves that regrew after injury end up cross-wired. Instead of the mouth-lifting muscles firing independently, they co-contract with the DAO, so trying to smile actually pulls the mouth corner down at the same time. This antagonism between the lifting and depressing muscles is a hallmark of post-facial-paralysis synkinesis and can be more distressing than the original palsy.5PubMed Central. Dynamic augmentation of lip levator function via depressor anguli oris transfer for post-facial paralysis synkinesis Treatment for synkinesis often involves selectively weakening the overactive DAO with botulinum toxin injections, sometimes combined with rehabilitation exercises, to restore a more balanced movement.6PubMed Central. Efficacy and Short-term Durability of Combined Botulinum Toxin Type A and Rehabilitation for Chronic Facial Asymmetry After Facial Nerve Palsy
Less commonly, autoimmune conditions can produce facial weakness that mimics nerve damage. Myasthenia gravis causes fatigable weakness of skeletal muscles, and the muscles of the face and around the eyes tend to be affected early.7PubMed Central. Assessing facial weakness in myasthenia gravis with facial recognition software and deep learning In rare cases it can look almost identical to Bell’s palsy, with unilateral drooping around the mouth as a presenting symptom.8PubMed Central. Myasthenia Gravis Masquerading as an Idiopathic Unilateral Facial Paralysis (Bell’s Palsy)-A Very Rare and Unique Clinical Find The key difference is that myasthenia gravis typically causes weakness that worsens with repeated use of a muscle and improves with rest, whereas Bell’s palsy weakness is constant. Any sudden facial droop warrants immediate medical evaluation because the differential diagnosis includes stroke, and speed matters in that scenario.
How Dental Changes Affect Mouth Shape
A cause that often surprises people is the loss of teeth or significant wear of the bite surface. When your back teeth are lost or worn down severely, the distance between your upper and lower jaw when your mouth is closed (what dentists call the vertical dimension of occlusion) decreases. The lower face essentially collapses inward, and the skin and muscle around the mouth fold over themselves, exaggerating the downturn of the corners.
This vertical dimension loss can arise from multiple sources: long-term grinding or clenching, acid erosion from conditions like gastric reflux, tooth loss without replacement, and even certain medications that dry the mouth and accelerate wear.9PubMed Central. Vertical Dimension in Prosthodontics Theory and Practice (Part III): Contemporary Clinical Protocols and Decision-Making for Loss of Vertical Dimension of Occlusion People who wear full dentures are particularly susceptible, especially if the dentures are old and the acrylic teeth have worn flat. Restoring proper bite height with new dental prosthetics or implants can noticeably reverse this component of mouth corner drooping, sometimes making a dramatic difference in facial appearance.
Drooping Mouth Corners in Newborns
Drooping at the mouth corner is not always an adult concern. Some newborns are born with a condition called asymmetric crying facies, where one corner of the mouth fails to pull down normally during crying while the other side works fine. Paradoxically, the affected side looks “normal” at rest and the unaffected side droops when the baby cries, creating an asymmetric expression. The cause is typically congenital underdevelopment of the DAO muscle on one side.10PubMed Central. The Manifestation of Congenital Hypoplasia of the Depressor Anguli Oris: A Case Report of Asymmetric Crying Facies
The condition is mostly cosmetic and tends to become less noticeable as the child grows, but pediatricians pay attention to it because it can sometimes be associated with other congenital abnormalities, particularly involving the heart. A baby with asymmetric crying facies usually gets a cardiac evaluation to rule out structural heart defects. In the vast majority of cases, no treatment is needed for the facial asymmetry itself.
How Drooping Corners Change the Way Others See You
Even a subtle downturn at the mouth corners significantly alters how other people read your face. A study on the effects of sleep deprivation on facial appearance found that observers consistently rated sleep-deprived faces as having droopier mouth corners, and those faces were also perceived as looking sadder.11Oxford Academic (SLEEP). Cues of Fatigue: Effects of Sleep Deprivation on Facial Appearance The sadness rating was directly linked to the perceived fatigue of the face, meaning that even temporary drooping caused by poor sleep was enough to shift social perception.
For people with permanently downturned mouth corners, whether from aging, muscle hyperactivity, or nerve damage, this social misreading can become a source of genuine frustration. Friends, colleagues, and strangers may ask “Are you okay?” or “Why do you look so upset?” when the person feels perfectly fine. Some dermatologists and plastic surgeons refer to the demand for treatment of this area as being driven as much by emotional misattribution as by aesthetic dissatisfaction. An overactive DAO that produces a “resting sad face” can affect how someone is treated in professional and personal settings, which is part of why treatment options for this specific concern have expanded considerably.
Treatment With Botulinum Toxin
The most common nonsurgical approach to lifting drooping mouth corners is injecting small doses of botulinum toxin (Botox and its relatives) directly into the DAO muscle. The idea is straightforward: weaken the muscle that pulls the corners down, and the upward-pulling muscles gain the upper hand, producing a subtle lift at rest. Studies confirm that this approach works, improving the appearance of drooping and reducing the impression of a sad or aged expression.12PubMed. Application of Local Injection of Botulinum Toxin A in Cosmetic Patients with Congenital Drooping Mouth Corner
The procedure sounds simple but requires precision. The DAO is a thin, fan-shaped muscle, and nearby muscles control the lower lip’s movement. Injecting too much toxin or placing it in the wrong spot can temporarily weaken the muscles responsible for pulling the lower lip down when you speak or eat, leading to an asymmetric smile or difficulty with certain mouth movements. Research into the safest injection sites has identified a triangular zone along the lower jawline as the area where the DAO is most reliably targeted with minimal risk of hitting neighboring muscles.2Plastic & Reconstructive Surgery. Anatomical Considerations Regarding the Location and Boundary of the Depressor Anguli Oris Muscle with Reference to Botulinum Toxin Injection The effect typically lasts three to four months before the muscle recovers and the injection needs repeating.
Fillers and Combined Approaches
When volume loss is a major contributor, as it often is in age-related drooping, fillers can address what botulinum toxin alone cannot. Hyaluronic acid fillers injected into the deeper layers around the mouth corners and along the marionette lines restore some of the lost scaffolding, physically supporting the tissue and smoothing the crease. Filler injections using a cannula for the deeper tissue layers have been shown to be effective for treating the volumetric component of the problem.13PubMed. Anatomical-based diagnosis and filler injection techniques: marionette line (static labiomandibular fold)
Combining fillers with botulinum toxin often produces a more complete result than either treatment alone. One study found that treating drooping mouth corners with hyaluronic acid filler combined with botulinum toxin improved the angle of the mouth corners from a median of about four degrees below horizontal at baseline to roughly one degree below horizontal, and the improvement held at three months.14PubMed. Nonsurgical correction of drooping mouth corners using monophasic hyaluronic acid and incobotulinumtoxinA A larger study evaluating a specific hyaluronic acid filler for marionette lines found that about 70% of participants had a clinically meaningful improvement at one month, with significant gains in patient-reported satisfaction.15PubMed Central. An Evaluation of VYC-17.5L for the Treatment of Marionette Lines: A Prospective, Open-Label, Postmarketing Study
Neither fillers nor botulinum toxin address the underlying bone loss or ligament laxity that drives much of the aging process, so these treatments are best understood as maintenance rather than permanent solutions. Most people who pursue them commit to repeat sessions on a schedule, with botulinum toxin every few months and fillers roughly once or twice a year depending on the product.
Surgical Options
For people who want a more lasting correction, or whose drooping is severe enough that injectable treatments produce only modest improvement, surgical procedures exist. The corner mouth lift is a targeted operation that removes a small wedge of skin just above the mouth corner and may also involve cutting or partially excising the DAO muscle to reduce its downward pull.16PubMed Central. Surgical Method of a Corner Mouth Lift The goal is to physically reposition the corner of the mouth a few millimeters higher and reduce the force dragging it back down.
Scarring is the main trade-off. The incision sits along the natural border of the lip, and skilled surgeons place it so the scar falls in the shadow line where the lip meets the surrounding skin. In some patients the scar fades well; in others it remains visible. Because the procedure is irreversible and the area is conspicuous, many practitioners recommend exhausting injectable options first and reserving surgery for cases where the degree of drooping exceeds what nonsurgical methods can correct.
In cases involving facial nerve paralysis, surgical interventions are more complex and may include muscle transfer procedures. One approach involves detaching the DAO from its normal position and repositioning it to assist the weakened lifting muscles, essentially converting a muscle that was pulling down into one that helps pull up.5PubMed Central. Dynamic augmentation of lip levator function via depressor anguli oris transfer for post-facial paralysis synkinesis These procedures are specialized and typically performed at centers with expertise in facial reanimation surgery.
Habits and Everyday Factors That Make It Worse
Beyond the medical and anatomical causes, several everyday factors can accelerate or exaggerate the appearance of drooping mouth corners. Chronic sleep deprivation is one. As the sleep study demonstrated, even acute sleep loss produces visible drooping at the mouth corners that other people perceive as sadness and fatigue.11Oxford Academic (SLEEP). Cues of Fatigue: Effects of Sleep Deprivation on Facial Appearance Over years, habitual poor sleep likely compounds the underlying tissue changes.
Sun exposure accelerates the breakdown of collagen and elastin in the skin, thinning the tissue that supports the mouth corners. Smoking does the same, with the added mechanical component of repeated pursing motions around the lips. Rapid or significant weight loss can deflate the fat compartments in the lower face faster than the skin can tighten, temporarily worsening the downturn. And habitual facial expressions matter: people who unconsciously clench their jaw or hold tension in their lower face may keep their DAO in a state of chronic contraction that, over years, contributes to the muscle becoming dominant.
None of these factors operate in isolation. A person who smokes, sleeps poorly, and has lost several back teeth will see the effects converge around the mouth corners in a way that no single intervention fully addresses. Understanding that drooping mouth corners are typically the visible endpoint of multiple simultaneous processes helps explain why the most effective treatment strategies tend to involve more than one approach.
Facial Muscles Across Human Evolution
An unexpected window into why the mouth corner area is so expressive comes from comparative anatomy. Research reconstructing the facial muscles of extinct human species has found that modern humans have proportionally smaller perioral muscles than our evolutionary relatives, including a smaller triangularis (the older anatomical name for the DAO). At the same time, modern human faces show greater development of the corrugator supercilii, the brow-furrowing muscle associated with complex emotional signaling.17bioRxiv. Muscles of Facial Expression in Extinct Species of the Genus Homo The authors suggest this reflects a trade-off: as human social communication shifted toward speech and more nuanced emotional displays, the facial muscles evolved to favor subtlety over raw mechanical power. Our relatively delicate perioral muscles give us an extraordinary range of expression, but they also mean the balance around the mouth corners can be thrown off more easily by aging, nerve damage, or tissue loss than it could in a face built for brute-force chewing.