If you fall sick this year, you may want to try drinking some green tea with honey, taking some quality sauna time or eating a nice, big cheese-and-raw-onion sandwich. These are just some of the home remedies for combating the flu that our readers came up with.

What effect do such DIY preventions and cures really have? That might depend on what you expect.

Dr Bruce Barrett has studied cold and flu treatments, including home remedies, and has looked closely at the placebo effect in his research. He says that most sick-day rituals – from homemade chicken soup to a few drops of Echinacea in your juice – can be relatively effective in shortening the severity of illness and symptoms... as long as you believe they're effective.

"The placebo effect, I think, goes way beyond placebo pills," Barrett said. "The theories evoked most commonly to explain it are those on expectations. There's a huge literature on that. When people expect things to happen, they'll happen more often."

Home remedies

We asked Dr Barrett to help us evaluate the home remedies that Guardian readers suggested last week, and explain the real science behind them.

First, Barrett explained that flu and the common cold are not as vastly different as we like to think. Despite being caused by different viruses the two illnesses manifest pretty indistinguishable symptoms in most people. That is why many of our readers' home remedies are classic cold "cures", but can be applied easily to mild symptoms of the flu.

Lucky for us, Barrett has written a textbook review of home remedies for all sorts of "viral upper respiratory infections". Here are some of the highlights:

• Chicken soup: The most iconic of cold and flu treatments is actually "somewhat supported" by human studies. Downing a bowl of real chicken soup has been shown to help un-stuff your nasal passages and help airflow. Other evidence of the medicinal value of chicken soup is disputed.

• Vitamin-C: Vit-C has been shown to have "some preventative effectiveness", though "no clear consensus exists to explain why some trials found benefit and others did not". Still, Vit-C may "reasonably" be assumed to have "small benefits on duration and severity" of respiratory illness if taken correctly – ie, 200-500 mg daily. But be careful – too much of the vitamin can cause diarrhea, stomach pain, nausea and heartburn.

• Saunas and hot showers: Inhaling "hot moist air, often with botanical and other additive" may help with stuffy nasal passages, depending on which trials you believe. But there has been no conclusive evidence that such interventions shorten sickness or placate symptoms. And do be careful: "Inhalation of vapors near [100°C or 212°F] may cause significant thermal damage."

Echinacea. If that doesn't work, hot toddy. And plenty of them. – Commenter TXfoodman

• Hot toddies: While there have been no well-known trials to test the medicinal effects of adding a bit of whiskey to your tea, this method of easing sickness seems to be a favorite for many, "including several physicians". But remember: all in moderation. This remedy should is not meant for kids, pregnant women and others who shouldn't be ingesting alcohol in the first place.

• Echinacea: This popular herbal remedy has disputed medicinal value. Early trials that looked into its power for prevention and treatment of respiratory infections were positive, but more recent studies have shown mixed results, including Barrett's own.

In 2011, Barrett and a team from the University of Wisconsin undertook a study on how Echinacea may affect cold severity and duration. They found no convincing evidence of a medicinal effect from the herb overall, but there was an odd twist in the findings. A subgroup of study participants who had rated themselves as having a strong belief in the effectiveness of Echinacea in a pre-study survey did display significant improvement. Their colds were "substantially shorter and less severe" than those suffered by their peers, regardless of which pill they were given.

Barrett concluded: "These findings support the general idea that beliefs and feelings about treatments may be important and perhaps should be taken into consideration when making medical decisions."

The placebo effect

Of course, the idea that beliefs and expectations have a lot to do with the effectiveness of treatment is nothing new – shamans and healers have been making people feel better without proven medicinal interventions for centuries. And remedies like chicken soup to combat a cold – or the flu – are ingrained in collective folklore.

Whether or not any of our readers' remedies may be medically effective, they certainly help ease cold and flu symptoms for large numbers of people.

As Jennifer Ackerman writes in Ah-Choo!: The Uncommon Life of Your Common Cold, "As early as 1933, Harold Diehl, dean of the University of Minnesota Medical School, gave inert lactose tablets as 'cold remedies' to 35 students with acute colds, and they reported being promptly freed of their cold woes. Since, then researchers have noted that people taking placebo in the early stages of a cold experience milder symptoms."

While Diehl may have had the weight of medical authority behind him, taking a home remedy that comes from a trusted source may exert similar results.

Real medical remedies for cold and flu are few and far between, Barret said. Anti-viral drugs may be useful in the beginning stages of true influenza infection, but "those medicines turn out not to be very effective". Anti-virals are only effective if taken within the first two days of illness, too.

In this light, indulging in whatever home remedy makes us feel better – from honey-cinnamon concoctions to onion sandwiches – might be the best answer for those who get the flu this season.

Conclusions

"Luckily the best medical advice for preventing flu is the best medical advice for preventing heart attack stroke and lots of other things," Barrett said. "And it's living a healthy life. We know that people who exercise regularly get less flu. People who eat well do too. People who don't smoke, people who don't drink to excess are at a much lower risk."

Flu shots are a good option too.

Barrett's bottom line is that "General health maintenance – psychological, mental, and physical – is the most important thing. Next probably would be flu shots."

But if you do fall ill, drink your hot toddies and take your hot showers – if you think they'll make you better, they just might.*

*Though, of course, do seek prompt medical attention if you take a turn for the worse or are at high risk for developing complications from the flu