Each winter, millions of people come down with the common cold, the flu, or another respiratory illness. Taking vitamins is a traditional remedy, but how well do they really work?

Science actually has a lot to say about this. Whether you are a fan of big pharma or not, vitamin, mineral, and herbal supplementation for health reasons is a booming business, and where there is money to be made, there is also research going on. Unfortunately, the numerous studies produced every year do not exactly compare apples to apples. Different studies may (often) use different strengths and preparations of the supplement in question. Ingredients in different brands sold over-the-counter are not standardized, and there is little to no requirement for quality control (at least in the United States, and in Australia products are only evaluated for safety, not effectiveness).  Different preparations and different brands can even be absorbed differently by the body – you may be taking something just to have it go right through you, so to speak (a doctor friend of mine talks about having very expensive pee). All of this can make it harder to determine whether a given supplement is actually going to help you stay healthy. But it’s better than no science at all, by a long run!

It’s also important to note that your health “starting point” will make a big difference in how well any of these supplements work, if they work at all. If you are deficient in a nutrient, then increasing your intake will almost guarantee you a stronger immune system and healthier body. This also goes for people who are under high levels of physical stress (think lots of exercise or a highly active job). These types of activity can depress your immune system, and recovery from often requires more nutrients than it would otherwise.

Now let’s see what the science has to say about some of the most common cold-fighting vitamins, minerals, and herbal supplements.

Vitamin C

Famous for being THE sickness-fighting vitamin, research results on the effectiveness of vitamin C have actually been somewhat controversial, for the reasons mentioned above. In normal healthy people, regular vitamin C supplementation in doses of up to 4g per day has shown a small reduction in the duration and severity of illness. There is not A LOT of evidence that it makes a person any less susceptible to infection in the first place (though there is some). To make matters more confusing, there is no agreement on how much you should take, or how often, for optimal effect. On the whole, the science supports the use of vitamin C, but they aren’t sure how much you actually need.

Zinc

Often sold and/or taken in tandem with vitamin C, large doses of zinc have been shown to prevent viruses from sticking to the mucus membranes of the body (think: sinus cavity) and from replicating. Daily supplementation of zinc in order to prevent colds is not recommended, since long term use can have adverse side effects, including suppressing the immune system – the exact thing you’re taking it to avoid! If you do come down with a cold though, there is evidence that zinc supplements may be useful in decreasing both the duration and severity of the illness. Optimal dose size and frequency have yet to be determined, so stick with the instructions on the packaging.

Echinacea

This is one of the most commonly sold herbal “cold-fighting” supplements. Best-selling does not always mean highly-effective though. A 2015 meta-analysis of research done on echinacea products suggested that some products may be associated with a decreased risk of catching a cold, but that there was little evidence that, once sick, these products would decrease the duration or severity of the illness. Interestingly, the individual studies included in the meta-analysis actually showed no association between supplementation and illness prevention. Another report from the Harvard Medical School agreed that evidence of echinacea fighting colds is weak. It still might work for you – if nothing else, there is always the placebo effect (if reading this didn’t ruin it). Maybe let the buyer beware?

Vitamin D

Vitamin D and sunshine are closely linked (sunlight helps the body produce some vitamin D internally). There is also more and more evidence that vitamin D and immune function are linked. Even people who spend significant time outdoors can be vitamin D deficient (and even in part of the world with great weather), prompting suggestions in some research that regular vitamin D supplementation be used to keep the immune system powered up. Your vitamin D levels can be detected in a blood test, and your doctor can help you determine what dosage is right for you. If you don’t feel like making a trip to the doc especially to get this checked, the recommended daily allowance (RDA) for vitamin D intake is 600 IU per day in both the United States and Canada.

Probiotics

The surprise cold fighter. Many people think of gastrointestinal support when they think of probiotics, and they aren’t wrong. It just so happens that we get a two-for-one with probiotics. Our digestive system is very closely linked to our immune system; much of the bacteria we encounter on a daily basis ends up going through our digestive tract and approximately 70% of the body tissue that secretes immune cells is located along the lining of our gut. Probiotics are actually live microorganisms that help your body out, often thought of as “good” bacteria. You can take them in capsule form, and almost every type of fermented food is rich with them (think: yogurt, kimchi, sauerkraut, probiotic drinks, etc.).  Regular probiotic intake – whether via food or a specific supplement – has been shown to decrease the risk of catching a cold to begin with, as well as decreasing the duration of illness.

Again, the results of this medical research don’t always give us clear-cut answers. For many supplements, the question of how much to take and how frequently hasn’t been clearly answered. Follow the instructions on the packaging, since each product can differ in strength, preparation, and effectiveness. Also remember that it can be difficult to control for placebo effects. For some people, simply the act of taking anything can be enough to give themselves both a physical and mental boost. And that is nothing to dismiss. If you are generally healthy (without any medical conditions or prescriptions that vitamin, mineral, or herbal supplements may interfere with), and you feel even better when you take these things, then by all means, stay healthy!