What causes tooth decay and gum disease?

Tooth decay and gum disease are two of the most common oral health problems, and they often develop slowly over time. Understanding what causes them is the first step in preventing cavities, bleeding gums, bad breath, and tooth loss.

How tooth decay develops

Tooth decay (dental caries or cavities) is the breakdown of tooth enamel caused by acids produced by bacteria in your mouth.

The role of plaque and bacteria

Your mouth naturally contains many kinds of bacteria. Throughout the day, these bacteria form a sticky film on your teeth called dental plaque. When you eat or drink anything containing sugars or starches, plaque bacteria feed on these carbohydrates and produce acid as a by-product.

Over time, these acids:

Dissolve minerals in the enamel (demineralization)
Create weak spots in the tooth surface
Eventually form a cavity (a permanent hole in the tooth)

If plaque isn’t removed regularly with brushing and flossing, it can harden into tartar (calculus), which strongly adheres to the tooth and can only be removed by a dental professional. Tartar makes it easier for plaque to stick and harder for you to clean effectively, accelerating tooth decay.

Diet and tooth decay

What you eat and how often you eat are major factors in cavity formation:

Sugary foods and drinks (soda, juice, energy drinks, candy, desserts) feed decay-causing bacteria.
Sticky or slow-dissolving snacks (caramel, dried fruit, chewy candies, crackers) cling to teeth and keep sugar on the surface longer.
Frequent snacking or sipping bathes teeth in acid over and over, without giving saliva a chance to neutralize acids and repair enamel.
Acidic drinks (soft drinks, sports drinks, flavoured waters, citrus juices) can erode enamel directly, making teeth more vulnerable to decay.

Dry mouth (reduced saliva)

Saliva helps protect teeth by:

Washing away food particles
Neutralizing acids
Providing minerals (like calcium and phosphate) to repair early enamel damage

When you have dry mouth (xerostomia), you lose this natural protection. Causes of dry mouth include:

Many common medications (for allergies, blood pressure, depression, anxiety, etc.)
Medical conditions such as Sjögren’s syndrome or diabetes
Cancer treatments to the head and neck
Dehydration
Mouth breathing or snoring
Tobacco and alcohol use

Without enough saliva, tooth decay can progress more quickly and affect more teeth at once.

Other risk factors for cavities

Several additional factors can increase your risk of tooth decay:

Poor or inconsistent oral hygiene (infrequent or ineffective brushing and flossing)
Deep grooves and pits in molars that trap food and plaque
Worn, broken, or defective fillings that collect plaque
Orthodontic appliances (braces, aligners) if not cleaned thoroughly
High sugar or refined carbohydrate diet
Young children with bottles or sippy cups (especially if filled with milk, formula, juice, or sweetened drinks at bedtime)
Medical conditions that affect diet, saliva or oral hygiene ability

How gum disease develops

Gum disease (periodontal disease) is an inflammatory condition that affects the tissues supporting your teeth. It typically starts as gingivitis and can progress to periodontitis if not treated.

Gingivitis: early stage of gum disease

Gingivitis is the mild, reversible form of gum disease. It is usually caused by plaque buildup along and under the gumline.

Common causes and contributing factors:

Plaque accumulation around the gums
Inadequate brushing and flossing
Tartar that irritates the gums and harbours bacteria

Signs of gingivitis include:

Red, swollen gums
Gums that bleed easily when brushing or flossing
Bad breath or a bad taste in the mouth
Tenderness along the gumline

At this stage, bone and deeper tissues are not yet damaged, and professional cleaning plus improved home care can usually reverse the condition.

Periodontitis: advanced gum disease

If plaque and tartar remain on the teeth, gingivitis can progress to periodontitis. At this stage, the inflammation affects deeper tissues, and the supporting bone around the teeth begins to break down.

Key features of periodontitis:

Gums pull away from the teeth, forming pockets that trap more bacteria
The body’s immune response and bacterial toxins damage bone and connective tissue
Teeth may loosen or shift, and in advanced cases may need to be removed

Risk factors for periodontitis include:

Ongoing plaque and tartar buildup
Smoking or vaping
Uncontrolled diabetes
Genetic predisposition
Stress and poor sleep
Certain medications that affect gums or saliva
Hormonal changes (pregnancy, puberty, menopause)
Immune system disorders

Shared causes of tooth decay and gum disease

Although tooth decay and gum disease affect different parts of the mouth, they share many underlying causes.

Poor oral hygiene

Not brushing and flossing effectively allows plaque to build up on tooth surfaces and around the gumline. This plaque:

Produces acids that cause cavities
Irritates gums and triggers inflammation

Effective daily care includes:

Brushing twice a day with fluoride toothpaste
Flossing or using interdental cleaners once a day
Cleaning along the gumline, not just the chewing surfaces

Diet high in sugar and refined carbohydrates

Frequent consumption of sugary or starchy foods and drinks:

Fuels acid-producing bacteria that cause decay
Contributes to an environment where harmful bacteria thrive along the gumline

Choosing water, fresh vegetables, whole fruits, dairy products, and high-fibre foods can help protect both teeth and gums.

Tobacco and vaping

Smoking and vaping significantly increase the risk of both tooth decay and gum disease by:

Reducing blood flow to the gums
Impairing immune response and healing
Increasing tartar buildup
Often contributing to dry mouth

Smokers are more likely to develop severe gum disease and to lose teeth. They may also have more root cavities due to gum recession and dry mouth.

Medical conditions and medications

Certain health issues impact both teeth and gums, including:

Diabetes: Poorly controlled blood sugar increases the risk and severity of gum disease and can slow healing, while infections in the mouth can make blood sugar harder to control.
Autoimmune disorders: Can affect gum health and dry mouth.
Medications causing dry mouth or gum overgrowth: These can alter the mouth’s environment and make hygiene more challenging.

Hormonal changes

Hormonal fluctuations can make gums more sensitive and prone to inflammation. This can occur:

During puberty
During pregnancy (pregnancy gingivitis)
Around menopause

When combined with plaque, these changes can increase the risk of gum disease and, indirectly, tooth decay along exposed root surfaces.

How to reduce your risk of tooth decay and gum disease

Understanding the causes means you can take specific steps to protect your oral health.

Daily home care

Brush twice daily with fluoride toothpaste for at least two minutes.
Use a soft-bristled toothbrush and gentle technique along the gumline.
Floss once a day to remove plaque between teeth and under the gums.
Consider interdental brushes, water flossers or other aids if you have braces, bridges or implants.
If you have dry mouth, sip water throughout the day, avoid constant snacking, and talk to your dentist about saliva substitutes or fluoride products.

Diet and lifestyle

Limit sugary and acidic foods and drinks, especially between meals.
Drink water instead of sweetened beverages; use a straw when you do have acidic drinks.
Avoid smoking and vaping.
Choose tooth-friendly snacks like cheese, nuts, raw vegetables and plain yogurt.
Rinse with water after eating if brushing isn’t possible right away.

Regular dental visits

Dental exams and professional cleanings are essential because:

Plaque and tartar are removed from areas that are hard to reach at home.
Early signs of tooth decay and gum disease can be detected and treated before they become more serious.
Your dentist or hygienist can recommend personalized products and techniques based on your risk level.

If you are covered under a plan such as the Canadian Dental Care Plan (CDCP) or another dental benefit program, regular preventive visits may be included—check your coverage and booking options with your dental clinic.

When to contact a dentist

See a dentist promptly if you notice:

Tooth sensitivity to hot, cold or sweets
Toothache or pain when chewing
Visible holes or dark spots on teeth
Bleeding when brushing or flossing
Red, swollen or receding gums
Persistent bad breath or a bad taste in the mouth
Loose or shifting teeth

Early care can often reverse or halt damage caused by tooth decay and gum disease, helping you keep your natural teeth healthy and comfortable for years.