A bad filling typically shows visible gaps, cracks, or dark discoloration at the edges where the filling meets your tooth. You might also notice the filling looks chipped, feels rough to your tongue, or has changed shape compared to when it was first placed. Some failing fillings are obvious at a glance, while others hide decay underneath that only shows up on an X-ray.
Visible Signs at the Edges
The most telling sign of a bad filling is what’s happening at its margins, the border where filling material meets natural tooth. A healthy filling sits flush against the tooth with no visible seam. A failing one develops small cracks, gaps, or a “ditch” running along the edge. These gaps form over time as the filling material shrinks, wears down, or loses its bond to the tooth. Even tiny spaces invite bacteria, saliva, and food particles underneath the restoration.
Marginal cracks are one of the strongest indicators of trouble. In a study of replaced fillings, roughly half had cracks smaller than 0.4 millimeters (barely visible to the naked eye), and the other half had cracks larger than that. Even the smaller cracks were significant: fillings with marginal cracks had an 86% chance of hiding decay underneath for amalgam (silver) fillings and a 64% chance for composite (tooth-colored) fillings.
Discoloration and Dark Lines
Brown or gray discoloration around a filling’s edges is another red flag. With tooth-colored composite fillings, you might notice a brownish shadow or line forming where the filling meets the tooth. This discoloration doesn’t always mean decay, but it does roughly 40% of the time. The staining can also come from coffee, tea, or other pigments seeping into micro-gaps at the margin.
Silver amalgam fillings darken naturally over the years, and the surrounding tooth can pick up a grayish tint from the metal. This discoloration alone isn’t necessarily a problem. What matters more is whether the edges look intact or whether you can see cracks, pitting, or separation between the filling and the tooth.
Chipping, Cracking, and Missing Pieces
Sometimes a bad filling is unmistakable because a chunk is visibly missing. You might feel a sharp edge with your tongue or notice a piece broke off while eating. In other cases, the entire filling falls out. If this happens without any pain, it usually means the bond between the filling and the tooth failed. This can happen when moisture contaminated the tooth surface during the original placement, or simply because the adhesive broke down over time.
A filling can also develop surface cracks that you can see if you look closely in good light. These cracks weaken the restoration and create pathways for bacteria. Composite fillings most commonly fail because of new decay forming at the margins, while amalgam fillings are more likely to fracture.
What a Bad Filling Feels Like
Not every failing filling is visible. Sometimes you’ll feel the problem before you see it. A filling that’s too high (interfering with your bite) causes pain when you clench or chew. This type of pain usually starts as soon as the numbing wears off after placement and persists with each bite.
Sharp sensitivity to hot or cold drinks is another signal. A brief zing that disappears once you remove the hot or cold stimulus suggests the filling is leaking but the nerve is still healthy. If the pain lingers for more than a few seconds after the temperature source is gone, that points to deeper nerve damage, which typically means the decay has progressed significantly.
Pain in a tooth with a composite filling is particularly telling. Research found that when a composite-filled tooth was painful, it indicated decay underneath the filling 100% of the time.
How Long Fillings Are Supposed to Last
Knowing a filling’s typical lifespan helps you gauge whether yours is due for trouble. Silver amalgam fillings last a median of about 16 years, while tooth-colored composite fillings last around 11 years. These are medians, so many fillings last longer, and some fail much sooner. A 2023 study found that 56% of replaced amalgam fillings were over 10 years old, compared to just 32% of composite fillings.
Composite fillings also carry a higher risk of developing new decay at the margins. One study found the risk of secondary cavities was 3.5 times greater with composite fillings than with amalgam. This doesn’t mean composite fillings are bad, but it does mean the edges deserve more attention as the filling ages.
What’s Happening Under the Surface
The most concerning scenario is a filling that looks fine on the surface but is hiding decay underneath. Gaps between the filling and tooth can form from polymerization shrinkage (the filling material contracting slightly as it hardens), air bubbles trapped during placement, or gradual dissolution of the filling’s edge in saliva. Bacteria colonize these spaces and cause secondary cavities that spread beneath the restoration where you can’t see them.
Dentists catch this hidden decay through X-rays. On a dental X-ray, recurrent decay appears as a dark shadow along the margins of the filling. This is why routine dental visits matter for filled teeth: a filling can look perfectly fine in the mirror while decay quietly spreads underneath it.
What Happens if You Ignore It
A failing filling exposes the inner layers of your tooth to bacteria. The progression is predictable: bacteria enter the gap, new decay forms, and that decay spreads deeper into the tooth. If it reaches the nerve, you’ll likely experience significant pain and may need a root canal instead of a simple filling replacement. Left longer, an abscess (a pocket of infection) can form at the root tip, potentially causing bone loss in the jaw and spreading infection to surrounding tissues.
The gap left by a broken or deteriorating filling also traps food and makes the area harder to keep clean, which can irritate the surrounding gums. Over time, this contributes to gum disease and, in severe cases, tooth loss. What starts as a simple replacement filling can escalate into a crown, root canal, or extraction if ignored long enough.
Checking Your Fillings at Home
You can do a basic visual check with a mirror and good lighting. Look for dark lines or brown shadows along the edges of your fillings, visible chips or missing pieces, and any obvious gaps between the filling and tooth. Run your tongue over the filling’s surface and edges. A healthy filling feels smooth and level with the surrounding tooth. Roughness, sharp edges, or a ledge you can catch with your tongue all suggest the filling has deteriorated.
Pay attention to how the tooth responds during meals. Sensitivity to sweets, lingering pain after hot or cold foods, or a dull ache when chewing are all signs worth investigating. If your filling is older than 10 years (especially if it’s composite), it’s worth having your dentist take a closer look at its margins even if nothing seems obviously wrong.