Should you drink coffee when you're sick?

For adults who already drink coffee every day, a cup during a cold is unlikely to dehydrate you, and a hot drink can ease cold symptoms. A stomach bug or a high fever changes the advice, and so do two medicines people often take when they're sick.

Coffee and hydration

Caffeine makes you urinate more, which is where the advice to avoid coffee when sick comes from. A 2014 crossover study in PLOS One tested that on 50 men who drank three to six cups a day. For three days they drank four 200 mL cups of coffee, dosed at 4 mg of caffeine per kilogram of body weight. For three other days they drank the same volume of water.

Total body water came out at 51.5 kg in the coffee trial and 51.4 kg in the water trial. Daily urine volume was 2,409 mL on coffee and 2,428 mL on water. The authors concluded that moderate coffee intake hydrates caffeine-habituated men about as well as water.

The subjects were healthy men used to caffeine. The study didn't include women, occasional coffee drinkers, or anyone losing fluid to illness.

Hot drinks and cold symptoms

At Cardiff University's Common Cold Centre, researchers gave 30 people with cold or flu symptoms an apple and blackcurrant cordial, served hot or at room temperature. The hot drink gave "immediate and sustained relief" from runny nose, cough, sneezing, sore throat, chilliness and tiredness, according to the university's summary of the 2008 Rhinology paper.

The drink in that study had no caffeine. A hot coffee gives you the heat and the steam. Nobody measured whether caffeine adds anything for cold symptoms.

Stomach bugs

Vomiting and diarrhea pull water out fast. The CDC's norovirus guidance says to drink plenty of liquids to replace it, and that over-the-counter oral rehydration fluids help most with mild dehydration. It names sports drinks and "other drinks without caffeine or alcohol" as options, which leaves coffee off the list.

The CDC's signs of dehydration in adults: you urinate less, your mouth and throat feel dry, or you get dizzy when you stand up. Until those signs pass, stick with water or a rehydration drink.

Medicines that interact with caffeine

The antibiotic ciprofloxacin "may increase nervousness, sleeplessness, heart pounding, and anxiety caused by caffeine," according to MedlinePlus, which tells patients not to have a lot of coffee, tea, energy drinks, cola or chocolate while taking it.

Pseudoephedrine, the decongestant in Sudafed, carries a similar note. MedlinePlus warns that foods and drinks with large amounts of caffeine can make its side effects worse, and those side effects include nervousness and a fast or pounding heartbeat.

The FDA says other medications can make people more sensitive to caffeine too, without listing them. Your pharmacist can check anything you've been prescribed.

Sleep

In a 2013 study in the Journal of Clinical Sleep Medicine, 12 healthy sleepers took 400 mg of caffeine, about two to three cups of coffee, at different times before bed. Taken six hours before bedtime, it cut total sleep by more than an hour on an in-home monitor. The participants didn't report the loss themselves. If you're sick and planning to sleep at 10pm, a 4pm coffee lands inside that window.

Fever

Mayo Clinic advises adults to call a health care provider at 103°F (39.4°C) or higher. It lists signs that need care right away when they come with a fever, including a stiff neck that hurts when you bend your head forward, trouble breathing or chest pain, persistent vomiting, and confusion.

Keeping the ritual with less caffeine

Decaf keeps the hot mug. It still has some caffeine: a 2006 University of Florida study measured 8.6 to 13.9 mg in 16-ounce cups of chain-store decaf, against about 85 mg in an 8-ounce cup of regular drip. Half regular and half decaf cuts the dose without changing your routine, and tea or broth gives you a hot drink like the one in the Cardiff study.

This post is general information, not medical advice. If you take a prescription or have a heart condition, ask your doctor or pharmacist about caffeine.