Why Antibiotics Don't Help a Cold, and When Bacteria Are Involved

It is day four of a cold, the mucus has turned green, and a half-used box of antibiotics still sits in the cupboard from last winter — the setting where unnecessary courses begin. Antibiotics act on bacteria, and colds and flu are caused by viruses, so a course will not shorten the illness or ease a single symptom. Bacteria do join in sometimes, but they announce themselves through a specific set of signs, never through the color of your mucus.

When to see a doctor

Right away, on your local emergency number — 911 in the US and Canada, 999 in the UK (111 for non-urgent advice), 112 in the EU and Russia, where 103 also reaches an ambulance.

For an adult: trouble breathing or shortness of breath; persistent pain or pressure in the chest or abdomen; persistent dizziness, confusion, or being impossible to wake; seizures; not urinating; severe muscle pain; severe weakness or unsteadiness; fever or cough that improved, then came back or got worse; a chronic condition flaring up (CDC, US).

For a child: fast or labored breathing with the ribs pulling in; bluish lips or face; chest pain; muscle pain so bad the child refuses to walk; dehydration — no urine for eight hours, a dry mouth, no tears; not alert or not interacting normally; seizures; a fever above 40 °C that will not come down; any fever in a baby under twelve weeks; improvement followed by worsening. The NHS (UK) adds a stiff neck, a rash that does not fade under a glass, discomfort with bright light, a first febrile seizure, pale, mottled, gray or bluish skin, drowsiness that is hard to break through, and extreme agitation. Neither list is exhaustive.

The same day, by appointment or phone: fever lasting more than four days (CDC, US) or more than three (NHS, UK); symptoms past ten days with no improvement; severe facial pain or headache; signs of dehydration. In a child: 38 °C under three months (NHS, UK), 39 °C between three and six months, fever for five days or more, a rash together with fever, refusing food and drink, dry diapers, sunken eyes.

Routine: a cough lasting more than three weeks; sinus infections that keep returning; a weakened immune system or a chronic illness — worth planning the season with your doctor.

What is happening

Antibiotics act on bacteria: they do nothing to viruses and do not make a cold milder or shorter (CDC, NHS). They are also not free: side effects run from a rash to serious reactions, and a needless course can still bring an allergic reaction or C. difficile diarrhea (CDC).

Mucus color is the most common false alarm: yellow or green discharge on the second or third day is a normal phase of a cold, not evidence of bacteria and not a reason for a prescription (CDC). The ordinary course of a cold says much more than the color.

Misuse of antibiotics is the main driver of resistance, and bacterial resistance was linked to more than 4.7 million deaths in 2021 (WHO). Every needless course makes the needed ones a little less reliable.

Bacteria do matter sometimes: a viral infection is occasionally followed by a bacterial one — pneumonia, for example — and then antibiotics are the right tool (CDC).

What to do

Treat symptoms, not the virus: rest, fluids and the measures on what helps with a cold; at the very start, the first signs of a cold.
Watch the calendar and the warning signs above, not the color of your tissues.
If a doctor does prescribe an antibiotic, take it exactly as prescribed, and never start a course from leftovers or someone else's prescription (CDC).
Be ready to be told to wait: for a middle ear infection a doctor may suggest watching for two or three days, or a delayed prescription to fill only if things do not improve (CDC).

When this is not the case

Sinusitis. Most are viral and settle without antibiotics over a few weeks (up to four, NHS, UK). The decision turns on severe headache or facial pain, symptoms that improved and then got worse, more than ten days without improvement, or fever lasting three to four days (CDC) — not on the color of the discharge.
Middle ear infection. Usually bacterial — pneumococcus or Haemophilus influenzae — yet the immune system often clears it alone. See a doctor for 39 °C or higher, discharge from the ear, symptoms lasting more than two or three days, or reduced hearing; a baby under three months with 38 °C needs help immediately (CDC).
Sore throat. Most are viral. Strep throat is bacterial and most common in children between five and fifteen: fever, pain on swallowing, red swollen tonsils with patches, and no cough or runny nose. It is confirmed by a rapid test or culture and does need an antibiotic, partly to prevent rheumatic fever (CDC).
Risk groups. People 65 and older, infants and children under five, anyone pregnant, and people with chronic lung, heart or kidney disease, diabetes, a weakened immune system, a disability with underlying conditions, or a BMI of 40 or more should see a doctor early rather than wait it out (CDC).

Sources

Checked: September 6, 2026