Sensitivity is the sharp pain you get from ice-cold water, hot coffee, or something sweet, in a tooth that has nothing wrong with it as far as you can tell. It is extremely common, it is treatable, and it is worth understanding, because a lot of the advice circulating about it is not very good.
What sensitivity actually is
Inside every tooth is a nerve. Around it are layers that protect it: the hard outer enamel on the crown, the cementum on the root, and beneath them the dentine, which is softer and contains fine tubules running through it.
In a healthy tooth, those layers seal the nerve off from the outside world. When the seal is breached, by wear, decay, gum recession, or age, the nerve receives stimulation it was never meant to receive, and it responds with pain.
The pain of sensitivity is short and sharp, and it stops when the stimulus stops. That is the diagnostic clue, and it is worth noting: if the pain lingers after the cold water is gone, that is a different problem and needs a dentist.
What causes it
Brushing too hard, or a brush that is too firm. This is one of the most common causes and one of the most commonly missed, because people who brush hard are usually trying hard to be doing the right thing. Enamel does not regenerate, so the wear is permanent.
The clue is sensitivity near the gum line, or a V-shaped notch appearing at the neck of a front tooth.
Grinding, or clenching. Many people clench without realising it, particularly at night. The forces are far greater than anything chewing produces. If your jaw is sore in the morning, or your partner has noticed noise, this is worth investigating.
Biting hard things contributes: ice, hard sweets, nuts, and using your teeth to open packaging.
Gum disease. With periodontal disease, the gum recedes and the root surface becomes exposed. Root dentine is not designed to cope with the outside world, and it is considerably more sensitive than enamel.
Decay. A cavity near the gum line exposes dentine directly.
Ageing. The enamel thins, the gums recede slightly, and the dentine tubules become more open. There is also, as a genuine factor, a reduced nerve response that makes older people more tolerant of the same stimulus. Sensitivity that appears later in life with no obvious cause is often simply this.
What to do about it
Start with the brush. A soft brush, light pressure, and no horizontal scrubbing. Most sensitivity caused by wear improves when the wear stops progressing, though what has already been lost does not come back.
Target the dentine. A desensitising toothpaste containing potassium nitrate, or a toothpaste with added calcium, works by reducing the nerve's response or by blocking the tubules. They usually need several weeks of regular use before you can judge them, which is where most people give up too early. For mild sensitivity, applying a little directly to the sensitive spot and leaving it may help, but follow the product instructions or ask your dentist.
Protect the teeth at night. Where grinding is involved, a night guard made by a dentist does more than any toothpaste.
Address the cause. Sensitivity caused by gum disease or decay does not resolve until the underlying problem is treated. This is the step people skip, and it is the reason symptoms return.
See a dentist if it is persistent. Where sensitivity is severe, or a tooth is clearly damaged, a dentist can treat the tooth itself: a filling, a bonded coating, a root canal treatment in severe cases, or a night guard. What they should not do is simply hand over a stronger toothpaste and send you on your way if the cause is treatable.
One thing to avoid
Sensitivity toothpaste that claims to stop bleeding gums is a different product with a different problem. Gums that bleed should be treated, not masked. A product that reduces visible bleeding can hide an important sign and make diagnosis harder rather than making the condition better.