Silver Diamine Fluoride (SDF): Why Would a Dentist Paint a Cavity Black?

"The dentist painted something on his tooth, and now it's black. Is that normal?" If your child has had silver diamine fluoride, the answer is usually yes. The dark colour is a sign that the treatment has reacted with the decay. SDF is not right for every cavity, but for the right child and the right tooth it can be a very useful, gentle option.

What is SDF?

Silver diamine fluoride is a clear liquid that contains silver, which helps kill the bacteria causing decay, and a high concentration of fluoride, which helps harden the weakened tooth. The dentist dries the tooth, then paints a tiny amount onto the cavity with a small brush. It takes a few minutes, and there is no drilling and usually no injection.

The aim is to arrest the decay: to stop it getting deeper and softer, so the cavity becomes hard and inactive. SDF does not rebuild the missing part of the tooth. A hole stays a hole, but often a hard, stable one.

The black stain: what to expect

• The decayed area turns black or dark brown, usually within a day or two. This stain is permanent on the treated area. Healthy enamel around it is usually not stained.
• On skin or gums, a small brown or grey mark may appear if the liquid touches them. This is temporary and usually fades within days to a couple of weeks.
• On clothes, SDF can leave marks that may not wash out, so the dentist will usually cover your child with a bib.
• Taste: it has a metallic, slightly bitter taste for a short time. Some children notice it, most cope well.

A dark spot on a back tooth is rarely noticed by anyone. On front teeth it is more visible, which is why the decision should be talked through together beforehand.

When SDF is often a good option

• Young children who are not yet ready to sit for a filling.
• Anxious children, or children with additional needs, for whom a long appointment would be hard.
• Buying time until a child is older, calmer, or ready for more complete treatment.
• Several cavities at once, to slow them all down while a plan is made.
• Cavities in baby teeth that will fall out fairly soon, where a more complex treatment may not be needed.

When SDF is not the right choice

• Deep cavities that are close to, or have reached, the nerve. SDF works on decay, not on an inflamed or dying nerve.
• A tooth with an abscess, a gum boil, swelling, or pain that wakes the child at night. These need a different treatment.
• An allergy to silver.
• Sore, ulcerated gums or mouth in the area being treated, until it has healed.
• Families who would be unhappy with a visible black stain, especially on front teeth.

The dentist will usually examine the tooth, and sometimes take an X-ray, before suggesting SDF.

Reapplication and follow-up

SDF often needs more than one application. Many dentists reapply it at follow-up visits, for example every six months or sooner, depending on how the cavity looks. At each check, the dentist confirms the decay has stayed hard and dark. If an area becomes soft again, or the tooth starts hurting, the plan may change.

Can it be covered later?

Yes. SDF does not close the door on other treatment. Once a child is ready, a tooth-coloured filling can often be placed over the treated area, although some darkness may still show through. A badly broken-down back tooth may later need a crown.

Helping SDF work at home

1. Brush twice a day with fluoride toothpaste, including the treated tooth. Brushing does not remove the treatment.
2. Follow the dentist's instructions after the visit. Some dentists ask that the child avoids eating or drinking for a short time.
3. Cut down how often your child has sugary snacks and drinks. SDF stops a cavity, but it doesn't prevent new ones.
4. Keep the follow-up appointments. Arrest is checked, not assumed.
5. Watch for warning signs: pain, swelling of the gum or face, or a pimple on the gum need a visit soon. Facial swelling with fever needs urgent attention.