General guidance — not a diagnosis. When in doubt, call your vet.
A new swollen eyelid in a dog should be checked by a veterinarian. The swelling may sit in the lid skin or at a small gland on its edge, but the eyeball can also be injured or painful underneath. Do not diagnose an allergy, insect sting, or stye by appearance alone.[1]
When to call right away
Seek urgent care if your dog holds the eye shut, squints hard, rubs it persistently, has a cloudy or protruding eye, or seems unable to see normally. Rapidly spreading facial swelling, hives with breathing difficulty, collapse, or a known bite or chemical exposure is an emergency. A lid that is swollen enough to prevent inspection of the eye should be assessed promptly. If your dog is comfortable and the eye looks clear, still arrange an examination soon, especially if the swelling grows or returns.
For dog owners
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Look at the lid and the eye separately
From a safe distance, note whether the lump is on the eyelid margin, within the lid skin, or below the eye. Compare it with the other side. A small focal bump can involve an eyelid gland; diffuse redness and crusting can reflect blepharitis. Inflammation, irritants, infections, and immune-mediated disease are among the possibilities.[1] A red or cloudy eyeball needs its own examination; the dog red-eye guide covers that distinction. Unequal pupils are a separate warning sign explained in the dog pupil guide.
Take a clear photo and record when swelling began, whether it is worsening, any recent outdoor activity or facial injury, and whether your dog rubs the eye. Do not squeeze a bump, pull the lid open against resistance, or put human eye drops, skin ointment, or steroid medicine near the eye. Non-ophthalmic preparations can irritate the cornea and conjunctiva.[1]
What the veterinarian may check
The veterinarian may inspect the cornea and conjunctiva, assess vision and eye comfort, and examine the eyelid margin and skin. Depending on the findings, they may use an eye stain or other eye tests, sample a skin lesion, or investigate an unusual or persistent lump. Some lid disorders need targeted treatment rather than a generic eye drop.[1][2] The canine eyelid-swelling workup outlines the clinician-facing localization.
Frequently asked questions
Could it be a bug bite? Yes, but a bite is only one possibility. Rapid facial swelling or breathing changes need emergency care.
Is a swollen eyelid the same as conjunctivitis? No. Conjunctivitis affects the membrane around the eye, and eyelid inflammation can coexist with other eye disease.
Can I use an old eye medication? No. The wrong product may irritate the eye or delay diagnosis, especially if the cornea is injured.
Sources
[1] Merck Veterinary Manual, Eyelids in Animals. https://www.merckvetmanual.com/eye-diseases-and-disorders/ophthalmology/eyelids-in-animals
[2] Merck Veterinary Manual, Diagnosis of Skin Diseases in Small Animals. https://www.merckvetmanual.com/integumentary-system/integumentary-system-introduction/diagnosis-of-skin-diseases-in-small-animals