Lockjaw (medical term: trismus) happens when the jaw muscles spasm or the jaw joint cannot move normally, which makes it hard or nearly impossible to open your mouth.

What causes lockjaw?

Lockjaw is a symptom, not a disease by itself. It can come from several different problems:

1. Temporomandibular joint (TMJ) problems

Conditions that affect the jaw joint and its muscles are one of the most common reasons for lockjaw.

  • TMJ disorders from arthritis, disc problems, or joint damage.
  • Clenching or grinding teeth (especially with stress) overworks and tightens the jaw muscles.
  • Malocclusion (when top and bottom teeth don’t line up well) can strain jaw and neck muscles, leading to stiffness and a “locked” feeling.

Imagine a door hinge with a worn‑out pad: if the hinge is irritated or the parts don’t line up, opening and closing becomes jerky or stuck. That’s similar to TMJ‑related lockjaw.

2. Muscle overuse, spasm, and stress

Jaw muscles behave like any other muscle in your body: overuse or constant tension can make them shorten, cramp, and spasm.

Common triggers include:

  • Excessive gum chewing or hard/chewy foods.
  • Nail biting, chewing on pens, or holding objects with your teeth.
  • Long periods of teeth clenching during the day or in sleep.
  • Poor posture and prolonged neck tension (e.g., hunching over screens).
  • High stress, anxiety, anger, or fear that makes you “hold” tension in your jaw.

When this happens, the muscles can’t fully relax, and your mouth cannot open normally.

3. Infections (mouth, throat, or jaw)

Local infections around the mouth and throat can irritate or mechanically restrict jaw movement.

Examples include:

  • Dental infections, such as infected or impacted wisdom teeth.
  • Tonsillitis or other throat infections.
  • Infected salivary glands.

Inflammation and pain make the muscles tighten reflexively, which can reduce how far you can open your mouth.

4. Tetanus (classic “lockjaw”)

Tetanus is a serious bacterial infection caused by Clostridium tetani , which releases a neurotoxin that affects the nervous system. A classic early sign is painful tightening of the jaw muscles—“lockjaw.”

Key points:

  • The toxin causes widespread muscle spasms, often starting in the jaw and neck.
  • It usually follows a wound contaminated with soil, dust, or manure if you’re not adequately vaccinated.
  • Without treatment, tetanus can be life‑threatening, but it is rare where vaccination is common.

Tetanus‑related lockjaw typically comes with other symptoms such as stiff neck, difficulty swallowing, and painful body spasms.

5. Trauma and dental procedures

Injury to the jaw area or certain treatments can cause temporary or lasting lockjaw.

  • Direct trauma: fractures, sports injuries, whiplash, or blows to the face can bruise muscles, inflame the joint, or alter its anatomy.
  • Dental work: long procedures with the mouth wide open or injections near jaw muscles can irritate or injure them, leading to short‑term trismus.

In these cases, pain, swelling, or mechanical blockage in the joint limits opening.

6. Cancer, radiation, and tumors

Problems related to cancer in the head and neck region can also cause lockjaw.

  • Surgery or radiotherapy for head and neck cancers can scar or stiffen the jaw and neck muscles.
  • Tumors in or near jaw muscles, TMJ, throat, larynx, thyroid, or salivary glands may impede normal jaw movement.

These causes tend to develop more gradually and may be accompanied by other concerning symptoms like weight loss, persistent pain, or lumps.

7. Medications and other less common causes

A few other medical factors can trigger jaw stiffness.

  • Certain psychiatric or neurologic medications can, rarely, cause muscle stiffness and jaw locking as a side effect.
  • Neurologic disorders that affect muscle tone and coordination can involve the jaw.
  • Rare structural or congenital jaw issues can also limit opening.

Is lockjaw an emergency?

Sometimes yes, sometimes no—it depends on the cause and symptoms.

Seek urgent or emergency care if you have lockjaw plus:

  • Trouble breathing or swallowing.
  • Fever, feeling very unwell, or spreading swelling in the face or neck.
  • Recent dirty wound or puncture plus not being up to date on tetanus shots.
  • Rapidly worsening stiffness or severe pain.

More routine—but still important—evaluation is needed if:

  • Your mouth opening is clearly reduced (e.g., you can’t fit three fingers vertically between your front teeth).
  • Jaw pain, clicking, or locking keeps recurring.
  • You’ve had recent dental work, injury, or new medications and notice new jaw stiffness.

Brief FAQ style recap

What causes lockjaw most of the time?
Most commonly TMJ disorders, jaw muscle overuse or spasm, and local dental or throat infections.

Is lockjaw always tetanus?
No. Tetanus is a classic but now rare cause where vaccination is common; many other jaw, muscle, or dental issues are more frequent causes.

Can stress alone cause lockjaw?
Stress does not damage the joint by itself, but it can drive clenching and muscle tension that lead to jaw spasm and temporary lockjaw.

Information gathered from public forums or data available on the internet and portrayed here.