Deep cavity reaching the nerve
The most common cause. Decay has eroded through enamel and dentin and is now within a fraction of a millimeter of the pulp, where it triggers irreversible inflammation. Often visible on x-ray.
Symptom Guide · 5 min read
Tooth sensitivity to cold that lingers — the 30-second rule
A short jolt of cold sensitivity is normal. Sensitivity that drags on after the ice water is gone is the single most reliable sign that something specific is happening inside your tooth — and it's worth knowing what.
Medically reviewed by Dr. Jason Kung, DDS, MS · Specialist Endodontist · UCLA DDS · OHSU MS ·
Reversible vs. irreversible — the test
You can do a rough version of the cold test at home with an ice cube wrapped in a thin paper towel:
- Pain stops within 5–10 seconds: Reversible pulpitis. The nerve is irritated but healing is possible. Address the cause — replace a leaky filling, treat the cavity, fix the gum recession — and the sensitivity usually resolves within weeks.
- Pain lingers 30+ seconds, or starts AFTER the cold is removed: Irreversible pulpitis. The nerve is past the point of self-recovery. A root canal is needed to save the tooth.
- Spontaneous throbbing pain (without a trigger), or pain that wakes you up: Late-stage irreversible pulpitis or early acute apical periodontitis. Schedule promptly.
- The tooth used to be cold-sensitive and suddenly stopped: Worth attention, not relief — the nerve may have died. Often followed weeks later by an abscess.
The "30-second rule" is a clinical heuristic, not an exact threshold. A tooth somewhere in the 10–30 second range is worth a specialist evaluation to characterize precisely.
What's causing it
Recent dental work irritation
Cold sensitivity following a recent deep filling, crown prep, or cleaning is normal — and usually resolves within 2–4 weeks. If it lingers past 4 weeks or progresses to spontaneous pain, the nerve has likely been overwhelmed and the tooth needs a root canal.
Cracked tooth
Cracks let cold liquid penetrate to the deeper layers of the tooth, triggering sharp, lingering sensitivity. Usually combined with sharp pain on biting. See our cracked tooth symptoms guide for the full pattern.
Leaking or failing filling
An old filling that has gapped or fractured at its margin lets bacteria and cold infiltrate underneath. Replacing the filling fixes the cold sensitivity if the nerve hasn't yet been irreversibly damaged.
Frequently asked questions
What does the '30-second rule' mean?+
It's a simple bedside test endodontists use to distinguish reversible from irreversible nerve inflammation. Apply something cold (ice cube, cold water) to the tooth and time how long the sensitivity lasts after the cold is removed. Less than 10 seconds: the nerve is healthy or only mildly irritated, and the tooth almost always settles down. 30 seconds or more — or pain that builds AFTER the cold is gone — strongly indicates irreversible pulpitis: the nerve cannot heal on its own and root canal treatment is the only way to save the tooth.
Could it just be sensitive teeth?+
Generalized sensitivity to cold across many teeth — especially at the gum line — is usually classic dentin hypersensitivity from gum recession or enamel wear. That responds well to a sensitivity toothpaste (potassium nitrate, Sensodyne) within 2–4 weeks. Lingering, focal, severe cold sensitivity isolated to one specific tooth is a different problem entirely.
I had a deep filling 2 weeks ago and it's sensitive to cold. Is something wrong?+
Probably not. About 1 in 5 patients have transient cold sensitivity for 2–4 weeks after a deep filling — the nerve is recovering from the heat and chemical irritation of the procedure. Watch for two warning signs: sensitivity getting worse rather than better, or new spontaneous pain (pain without a trigger). Either of those means the nerve has crossed into irreversible pulpitis and needs evaluation.
Can a sensitive tooth heal on its own?+
Yes — if the inflammation is reversible. The pulp has limited but real healing capacity when the irritant is removed (cavity filled, leaking restoration replaced, gum recession stabilized) and the inflammation is caught early. Once the inflammation crosses the irreversible threshold (the 30-second rule above), spontaneous healing is no longer possible.
Will I need a root canal?+
If the cold sensitivity has progressed to lingering pain, spontaneous pain, or pain that wakes you at night, then yes — root canal treatment is the only way to save the tooth. If you're caught it early (brief sensitivity that's still resolving in seconds), addressing the underlying cause (filling, crown, gum care) may be enough.
What about heat sensitivity?+
Heat sensitivity that lingers — especially if hot drinks make a tooth throb — is an even more specific indicator of irreversible pulpitis than cold sensitivity. Some patients describe relief from cold water (which constricts the blood vessels and reduces pressure), which is a textbook diagnostic clue.
Find out what stage your tooth is in
A 30-minute specialist consultation tells you exactly where the nerve stands. If we can avoid a root canal, we'll tell you. If we can't, we'll explain why.