One swollen tonsil is usually caused by something benign. In roughly 82% of cases, unilateral tonsil enlargement traces back to normal anatomical variation, a recent infection, or reactive tissue growth rather than anything dangerous. That said, the pattern of swelling, how long it lasts, and what other symptoms come with it all matter in narrowing down the cause.
Why One Tonsil Can Look Bigger Than the Other
Your two tonsils don’t have to be the same size. The pocket each tonsil sits in, called the tonsillar fossa, varies in depth from one side to the other. A shallower pocket on one side pushes that tonsil forward, making it look larger even though it isn’t. This is purely structural and completely harmless.
A study of 792 patients who had their tonsils removed found that among 53 people with asymmetric tonsils and no other risk factors, not a single case of malignancy or unusual pathology turned up on examination. The vast majority showed reactive lymphoid hyperplasia, which is a normal immune response where the tissue temporarily grows in reaction to germs. So if one tonsil has always looked a bit bigger and you have no other symptoms, the explanation is likely anatomical or related to your immune system doing its job.
Infection: The Most Common Cause
A viral or bacterial infection is by far the most frequent reason one tonsil suddenly swells. You might have a garden-variety sore throat that happens to hit one side harder, or one tonsil may have been fighting off a pathogen more aggressively. Viral tonsillitis typically clears up on its own within about a week, with most symptoms fading in three to four days. If swelling lingers past four days without improvement, that’s worth a closer look.
Recurrent bouts of tonsillitis can also leave one tonsil permanently larger than the other. Repeated infections cause scar tissue and chronic inflammation that change the shape and size of the tonsil over time. This is benign but can make it harder to tell whether new swelling is actually new.
Peritonsillar Abscess
A peritonsillar abscess is a pocket of pus that forms next to one tonsil, usually as a complication of untreated or undertreated tonsillitis. It causes dramatic one-sided swelling and has some distinctive signs: difficulty opening your mouth (because the inflammation spreads to the jaw muscles nearby), a visibly swollen uvula pushed to the opposite side, and significant throat pain that tends to be much worse on the affected side. Swallowing becomes painful, and your voice may sound muffled or “hot potato”-like.
This is not a wait-and-see situation. A peritonsillar abscess needs to be drained and treated with antibiotics. If you have severe one-sided throat pain, trouble opening your mouth, and a fever, you should be seen the same day.
Tonsil Stones
Tonsil stones are small, hardened deposits of bacteria, food debris, and dead cells that get trapped in the crevices of your tonsils. They look like tiny white or yellow pebbles. Most are small and cause nothing more than bad breath or a mildly annoying sensation. But larger or multiple stones on one side can make that tonsil look swollen and can cause difficulty swallowing.
Tonsil stones can also trigger referred ear pain. The tonsils share a nerve supply (via the glossopharyngeal nerve) with parts of the middle ear. Irritation on one tonsil can travel along this nerve pathway and show up as an earache on the same side, even though nothing is wrong with the ear itself. This same mechanism applies to any source of one-sided tonsil inflammation, not just stones.
When Swelling Could Signal Something Serious
Tonsil cancer and lymphoma can both present as a single enlarged tonsil. These are uncommon, but there are specific warning signs that set them apart from infections and benign causes. The key red flags include:
- Progressive enlargement: the tonsil keeps growing over weeks rather than shrinking
- A lump in the neck: a painless, firm mass on the same side
- Persistent difficulty swallowing or a feeling that something is stuck in your throat
- Unexplained ear pain on the same side that doesn’t go away
- Jaw stiffness without an obvious infection
HPV-related tonsil cancers, which have become increasingly common, often show up differently than other types. A JAMA study found that 51% of HPV-positive patients first noticed a neck mass rather than throat symptoms, compared to only 18% of HPV-negative patients. HPV-negative cancers were more likely to cause sore throat and painful swallowing early on. This means an unexplained neck lump paired with a swollen tonsil deserves prompt evaluation, even if your throat feels relatively normal.
Certain deeper growths, like tumors of the parotid gland or nerve-related masses, can push a tonsil inward from behind, creating the appearance of a swollen tonsil when the tonsil itself is actually normal. These are rare but important for a clinician to consider during an exam.
How Doctors Evaluate a Single Swollen Tonsil
The evaluation starts with your symptom history: how long the swelling has been there, whether it’s getting bigger, and what other symptoms you have. A physical exam of the throat, neck, and lymph nodes is usually enough to identify infections and abscesses.
Tonsil asymmetry alone, without other concerning features, generally does not require a biopsy. The research supports this clearly: in patients with asymmetric tonsils but no suspicious appearance, no progressive growth, no neck lumps, and no systemic symptoms like unexplained weight loss or night sweats, the risk of finding malignancy is extremely low. Biopsy or removal is typically reserved for cases where the tonsil looks abnormal (ulcerated, discolored, or firm), is growing steadily, or is accompanied by enlarged lymph nodes in the neck.
What the Timeline Tells You
Timing is one of the most useful clues. Swelling that appears suddenly alongside a sore throat and fever points to infection. If it resolves within a week or so, that’s confirmation. Swelling that appeared weeks or months ago and hasn’t budged, or that’s slowly getting larger, tells a different story and warrants a medical evaluation.
If you recently had a cold or throat infection and one tonsil is still slightly larger than the other a few weeks later, that’s common. Tonsil tissue can take longer to fully shrink back down than the rest of your symptoms take to resolve. But if the asymmetry persists beyond several weeks, or new symptoms like ear pain or a neck lump appear, that’s the point to get it checked.