Plaque consists of bacteria that attach to the tooth surface. When plaque and tartar stay close to the gum line, the inflammation can spread deeper around the tooth and damage the tissue that holds it in place. A bad smell is therefore often more than a cosmetic issue.
Look for red or bleeding gums, obvious tartar, loose teeth and whether your dog chews differently. Many dogs keep eating despite pain in the mouth. A proper assessment of the gum pockets and tooth roots is done under anaesthesia and is often combined with dental X-rays. Tooth brushing at home works best as prevention, not as treatment of established periodontal disease.
Book a dental examination if the smell is clear or persistent, or if your dog shows other signs of mouth pain. A sudden strong smell together with swelling or reluctance to eat may need to be assessed sooner.