What causes it
Sensitivity happens when the dentine underneath the enamel is exposed. That can be from gum recession leaving the root surface bare, enamel worn thin by grinding or by acidic food and drink, a cracked tooth, or decay. Sometimes it follows recent treatment and settles by itself within a few weeks.
Why the cause matters
Desensitising toothpaste helps mild cases and is a reasonable first step. But sensitivity is a symptom, not a diagnosis. If it is coming from a cracked tooth or decay, toothpaste only masks it while the real problem gets worse. That is why it is worth having it looked at rather than managing it indefinitely.
What we can do
Depending on what we find: fluoride varnish to strengthen the exposed surface, sealing the sensitive area, a filling where enamel has worn through, or treating the gum condition causing the recession. If you grind your teeth at night, a guard protects the enamel that remains.
Common questions
Does sensitive toothpaste actually work?
For mild sensitivity from exposed dentine, yes, though it takes a few weeks of consistent use. It will not fix a crack or a cavity.
Why is it worse with cold than heat?
Cold moves fluid inside the dentine tubules faster, which triggers the nerve. Pain that lingers with heat is more concerning and may mean the nerve is inflamed.
Could brushing harder help?
The opposite. Hard brushing wears enamel and pushes the gum back, which is a common cause. Use a soft brush and light pressure.