Mouth pain in a small child is one of those symptoms that pulls everything else with it — they won't eat, won't drink, drool more than usual, sleep badly, and look generally miserable. It's alarming when you can't see what's wrong because they won't open their mouth.
Two infections account for most cases in under-threes: herpetic stomatitis (a first-time herpes simplex infection) and oral thrush (Candida). They look different, behave differently, and need different treatment. This piece walks through how to tell them apart, what to do at home, and the points at which you should be calling your GP. Healthbooq covers common childhood infections and how to handle them safely at home.
Herpetic Stomatitis (HSV-1)
Most children meet the herpes simplex 1 virus for the first time somewhere in the first few years of life — usually from a parent or relative who carries it without knowing, often passed via a kiss. The first infection (called primary herpetic gingivostomatitis if you want the textbook name) is more dramatic than the cold sores that show up in adulthood.
The pattern is fairly predictable. It starts with a couple of days of fever, irritability, and a generally miserable child. Then small painful ulcers appear — on the gums, tongue, inside of the cheeks, lips, and the roof of the mouth. They are shallow with red edges, often clustered, and they hurt enough that the child will refuse anything that touches them. Drooling increases because swallowing hurts. Lymph nodes in the neck swell up. The whole thing lasts 7 to 10 days and the ulcers heal without scarring.
The main job during this week is keeping fluids in. Eating drops off and that's expected — energy reserves can ride out a week of low intake. Dehydration is the real risk, and it can creep up on a toddler faster than parents expect.
What helps:
- Paracetamol or ibuprofen, given on time, ideally 30 minutes before drinks. Adequate pain relief before fluids is the single biggest lever you have for getting them to drink.
- Cold and smooth wins. Ice lollies, cold milk, yoghurt, smooth mashed potato, custard. Cold numbs and smooth doesn't catch on ulcers.
- Avoid acidic and salty. Citrus, tomato sauce, crisps — they sting on contact and turn drinking into a battle.
- A straw can help if the ulcers are mainly on the tongue and lips, by routing fluid past the painful areas.
- Aciclovir (an antiviral) within the first 72 hours can shorten and soften the illness. It's not routinely prescribed for healthy children with mild disease, but ask your GP if your child is severely affected, very young, or has eczema (HSV in eczema can spread badly).
Signs of dehydration that mean the same-day GP or out-of-hours service:
- No wet nappy in 6 hours (or no urine in 8 hours for an older child)
- Dry mouth, no tears when crying
- Sunken eyes or, in babies, a sunken fontanelle
- Unusual drowsiness or floppiness
- Fast breathing or fast heart rate
Some children end up needing IV fluids — it's not rare. Don't tough it out at home if your gut says they aren't drinking enough.
A practical note: HSV-1 spreads through saliva. While your child has active mouth ulcers, no kissing on the mouth, no sharing cups or cutlery, and any cold-sore-prone adults in the household should be careful. The virus also tracks under fingernails — the herpetic whitlow that sometimes follows is a painful blistered fingertip from your child sucking on a finger that touched the sores.
Oral Thrush (Candida)
Thrush looks completely different. Instead of painful ulcers, you see white or creamy patches stuck to the tongue, inside of the cheeks, gums, or palate. The key test: try to wipe one off gently with a clean finger or gauze. Milk residue wipes away clean. Thrush sticks, leaves a raw red patch underneath, and may bleed slightly if you push it.
It's a normal mouth fungus that has overgrown — usually triggered by something that disturbed the balance: a recent course of antibiotics, an immune issue, or a feedback loop with maternal nipple thrush in breastfeeding pairs. Some babies feed normally with thrush; others fuss and pull off. If breastfeeding mum is getting shooting nipple pain, sore deep breast pain after feeds, or red shiny nipples, that's nipple thrush and the two need treating together — otherwise the cycle just keeps reinfecting.
Treatment is straightforward:
- Miconazole oral gel (Daktarin) for babies over four months and toddlers — applied to the patches after feeds. Use a clean finger; rub a small amount onto the affected areas.
- Nystatin oral drops for under-fours — same idea, given after feeds.
- Continue for 48 hours after the patches have cleared. This is the bit most parents skip and it's the bit that causes recurrence.
- If breastfeeding, the mother needs antifungal cream on her nipples (typically miconazole cream) at the same time, applied after feeds and continued for the same duration.
Thrush that doesn't respond to a full course, or keeps coming back, is worth a GP check — sometimes it points to an underlying issue, and sometimes it just needs a different antifungal.
Aphthous Ulcers (Canker Sores)
The third pattern, less common in under-threes but worth mentioning: a single round ulcer with a grey-white centre and a red halo, usually inside the lip or cheek, painful but not feverish. These aren't infectious, aren't contagious, and aren't from anything you did. Common triggers are minor trauma (biting the cheek, a hard rusk), stress, and occasionally nutritional issues.
They heal in 7–14 days on their own. A topical anaesthetic gel like Bonjela can help with eating. The things that warrant a GP visit: ulcers that recur frequently, are unusually big (>1 cm), last more than three weeks, or come with other symptoms — those can be the first sign of conditions like coeliac disease or inflammatory bowel disease and merit a closer look.
When to Call the GP
For any of these:
- Signs of dehydration with herpetic stomatitis (see list above)
- Suspected herpetic stomatitis in a baby under three months — needs same-day review
- A child who is significantly unwell, lethargic, or not responding normally
- Thrush not clearing on a full course of antifungal treatment
- Recurrent or persistent mouth ulcers
- Any child with a known immune condition or on immunosuppressants who develops mouth lesions
Key Takeaways
Stomatitis means inflammation of the lining of the mouth. In young children, the two common causes are first-time herpes simplex infection (herpetic stomatitis — painful ulcers, fever, refusal to eat) and oral thrush (Candida — white patches that don't wipe off). They look different and need different treatment. With herpetic stomatitis, dehydration is the main thing to watch for — eating barely matters for a week, drinking does. Oral thrush is straightforwardly treated with antifungal gel or drops; if breastfed, mum's nipples often need treating too.
