Pink Eye and the Acute Red Eye: Common Causes Versus Dangerous Ones

The short answer#

Most pink eye (conjunctivitis) is viral or allergic, not bacterial, and most cases clear on their own within one to two weeks. In adults, watery discharge with a gritty feeling and a recent cold points to viral; thick pus that glues the lids shut points to bacterial; itch in both eyes with a history of allergies points to allergic. The more important question is not which germ it is, but whether this is simple conjunctivitis at all. Real pain, true light sensitivity, or any drop in vision means the red eye needs urgent, in-person care, because those features suggest a deeper, sight-threatening cause.

Key points#

• Conjunctivitis comes in three common forms: viral (the usual cause in adults, most often adenovirus), bacterial, and allergic. No single symptom reliably separates viral from bacterial.
• Watery discharge with a red, gritty eye and a recent upper-respiratory illness suggests viral; thick pus with lids stuck shut in the morning suggests bacterial; itchy, watery eyes on both sides with hay fever or eczema suggest allergic.
• Most bacterial conjunctivitis resolves without antibiotics. In pooled trials, roughly 55 percent of untreated people improved within about a week, rising to about 68 percent with antibiotic drops.
• Antibiotic drops speed recovery only modestly, so for a mild case in an otherwise healthy adult they are optional; contact lens wearers, severe cases, and vulnerable eyes are handled differently.
• Allergic conjunctivitis responds to antihistamine or mast-cell-stabilizing drops, cool compresses, and avoiding the trigger. It is not contagious.
• Warning signs that are not simple pink eye: significant eye pain, light sensitivity, blurred or reduced vision, a hazy cornea, a white spot on the cornea, or eye pain in anyone who wears contact lenses.
• Handwashing and not sharing towels matter more than any drop for the viral and bacterial forms, which spread easily by hand-to-eye contact.

Is my pink eye bacterial, viral, or allergic?#

Honestly, the overlap is wide, and clinicians accept that no single sign settles it at the bedside. Still, the pattern of symptoms shifts the odds. Viral conjunctivitis in adults tends to bring watery discharge, a red and gritty eye, and often a cold, sore throat, or a tender lymph node just in front of the ear. It frequently starts in one eye and moves to the second a few days later. Bacterial conjunctivitis classically produces thick, mucopurulent discharge that mats the lashes and can seal the lids shut overnight. Allergic conjunctivitis is dominated by itch, is almost always in both eyes at once, and travels with other allergic patterns such as sneezing or eczema. It flares with contact with pollen, dust, animal dander, or eye cosmetics rather than with any infection.

This table sits within our broader skin, musculoskeletal, and eye health library, and it is a guide to probability, not a diagnosis. If the eye is only mildly red and irritated with no discharge, consider that not every red eye is an infection at all: chronic irritation from screens and low tear film is common and is covered in our companion piece, Dry Eye and Screens: What Helps, What Is Known, and When to Get Care.

What does the antibiotic-drop evidence show for bacterial conjunctivitis?#

The best summary is a Cochrane systematic review that pooled 21 randomized trials with more than 8,800 participants. It found that bacterial conjunctivitis is largely self-limited. About 55 percent of people given placebo drops had cleared by roughly day 4 to 9, compared with about 68 percent of those given antibiotic drops. That is a real but modest benefit: the risk of still having an infection at the end of therapy fell by roughly a quarter with antibiotics.

Put in absolute terms, the difference is around 13 people per 100 who recover a little sooner. That works out to a number needed to treat of roughly 7 to 8, meaning about seven or eight people take antibiotic drops for one extra person to be cleared faster than they would have been anyway. Serious complications were rare in both groups. The certainty of this evidence was rated moderate, and many trials were funded by drug manufacturers, so the true effect may be smaller.

What this means in practice matches current family-medicine guidance: for a healthy adult with mild symptoms, a shared decision between watchful waiting and a short antibiotic course is reasonable, and delayed prescribing (a prescription to fill only if things have not improved in a few days) gives similar symptom control to treating immediately. The calculus changes for specific groups. Contact lens wearers, people with marked pain or vision change, recent eye surgery, a weakened immune system, or copious rapidly worsening pus are treated more assertively and usually need to be examined rather than managed by phone.

How do you treat pink eye at home, and how long is it contagious?#

For adults, most of the work is comfort and hygiene. Cool compresses, artificial tears, and gently wiping discharge from the lids help all types. Viral conjunctivitis has no specific cure and simply runs its course over one to two weeks, sometimes with symptoms peaking around day three to five before easing. Allergic conjunctivitis responds best to an over-the-counter antihistamine or mast-cell-stabilizing eye drop, cool compresses, and removing the trigger; the itch is the giveaway. If bacterial conjunctivitis is treated, a topical antibiotic drop or ointment is typical.

A few practical rules matter more than the choice of drop. Stop wearing contact lenses until the eye is fully better and discard the current pair, case, and solution. Do not share towels, pillows, or eye makeup. Wash hands often, because viral and bacterial pink eye spread readily by hand-to-eye contact and can pass to household members. The contagious period for viral conjunctivitis can last as long as the eye is red and weeping, often a week or more, which is why staying home while the eye is actively discharging is sensible. Bacterial conjunctivitis is generally considered much less contagious after about 24 hours of antibiotic drops. Return-to-work and return-to-school rules vary by employer and locale, so check the specific policy rather than assuming.

One caution: reaching for a leftover steroid eye drop from a friend or an old prescription can be genuinely harmful. If the real problem is a herpes infection of the cornea, a steroid can let the virus flourish and threaten sight. Eye drops that combine an antibiotic with a steroid are not a self-care item.

Which red-eye causes are dangerous, not just pink eye?#

Conjunctivitis is a nuisance. The reason a red eye deserves respect is the short list of causes that can take vision within hours to days. These share a different flavor: pain that is more than gritty, genuine sensitivity to light, and blurred or reduced vision that does not clear when you blink.

• Acute angle-closure glaucoma: a painful, red eye with a mid-sized pupil that does not react, a hazy cornea, halos around lights, headache, and sometimes nausea and vomiting. This is an emergency. The mechanics of eye pressure are covered in our companion piece, Glaucoma Basics: Risk, Detection, and Protecting Vision.
• Keratitis and corneal ulcer: infection or inflammation of the cornea, marked by pain, light sensitivity, and often a visible white or gray spot. Risk is much higher in contact lens wearers, in whom pain plus lens intolerance is a warning to be seen the same day. Herpes simplex keratitis is a leading infectious cause of corneal blindness and is exactly the case where a steroid drop can make things worse.
• Iritis and uveitis: inflammation inside the eye, with a deep ache, light sensitivity, and redness concentrated in a ring around the colored part of the eye. It often ties to autoimmune conditions.
• Endophthalmitis: infection inside the eyeball, typically after surgery, an injection, or a penetrating injury. Pain and falling vision after any eye procedure is an emergency.
• Hyperacute bacterial conjunctivitis: a sudden flood of thick pus, most often gonococcal, which can invade the cornea quickly and needs systemic treatment, not just drops.

Allergic eye symptoms are almost always local and benign. But if red, itchy eyes come with lip or tongue swelling, hives, or trouble breathing, that is a different and urgent picture; see Anaphylaxis and severe allergic reactions.

When to seek care#

Get same-day or emergency eye care for any of these red flags:

• Moderate to severe eye pain, not just grittiness.
• True light sensitivity that makes you want to keep the eye shut.
• Any blurred vision or loss of vision that does not clear when you blink.
• A white spot, cloudiness, or haze on the cornea, or halos around lights with headache, nausea, or vomiting.
• Eye pain or worsening redness in anyone who wears contact lenses.
• Redness after recent eye surgery, an eye injection, or a penetrating injury.
• A copious, rapidly worsening flood of pus, or symptoms in a newborn.
• A cold sore or a blistering rash near the eye or on the tip of the nose along with the red eye.

For ordinary pink eye without these features, watchful waiting with hygiene and comfort measures is reasonable, with a check-in if things are not improving after about a week or are clearly getting worse.

It cannot diagnose your eye. If you have any of the warning signs above, or you are unsure, see a clinician or eye specialist in person.