Americans spent almost $3 billion on vitamins, herbal remedies, and other supplements in their fight against colds and flu in 2016, according to the Nutrition Business Journal.

But research showing that the products work is usually weak. And unlike drugs, the Food and Drug Administration doesn’t require that supplements be proved safe and effective before they head to market, and you can’t even be certain that they contain what their labels claim.

Here’s our take on several common ones:

SPECIAL REPORT: HOW TO TREAT A COLD OR THE FLU

Echinacea
What the research says: A 2014 review of 24 trials hinted that echinacea teas or supplements might help prevent colds.

Is it safe? Echinacea can trigger nausea and in some people worsen asthma.

CR’s take: Any hot tea can help soothe cold and flu symptoms. But it’s probably not worth regularly taking echinacea supplements on the off chance that they might help prevent colds.

Garlic
What the research says: Garlic extract supplements might help prevent colds, according to a 2014 review.

Is it safe? Frequent high doses may increase the risk of bleeding when combined with warfarin (Coumadin) and make HIV drugs less effective.

CR’s Take: No harm eating garlic. But do you really want to regularly pop garlic pills, hoping they might ward off colds?

Homeopathic Remedies
What the research says: The theory behind these products—which usually bear the word “homeopathic” on their labels­—is especially dubious. It involves taking a presumed active ingredient—such as oscillococcinum, an extract of wild duck heart and liver—and diluting it to the point that it’s imperceptible. The Federal Trade Commission says that “homeopathic product claims are not based on modern scientific methods.” Yet the products are often sold in stores alongside FDA-approved drugs. (Read more about homeopathic medicine.)

CR’s Take: Don’t bother.

Probiotics
What the research says: “Good bacteria,” best known to help foster a healthy gut, may also help prevent respiratory tract infections, according to a 2015 analysis.

Is it safe? The pills can sometimes cause bloating and gas and may lead to infections in people with seriously weakened immune systems.

CR’s Take: Consuming yogurt and other probiotic-rich foods is fine, but there’s not enough evidence to justify taking probiotic pills to fight colds or flu.

Vitamin C
What the research says: People who take 200 mg of the vitamin every day might have slightly shorter colds than other people, according to a 2013 review of 29 trials. But loading up after symptoms appear doesn’t seem to help at all.

Is it safe? The pills make kidney stones more likely, and high doses can cause nausea and diarrhea. They may also interact with cancer drugs, blood thinners, and estrogen.

CR’s Take: A better bet: Regularly include vitamin C-rich foods, such as greens and citrus fruits, in your diet.

Zinc
What the research says: A 2015 analysis found that zinc lozenges and syrup reduced the length and severity of colds when started within 24 hours after symptoms start.

Is it safe? Possible side effects include diarrhea, nausea, stomach cramps, and vomiting. Long-term use, especially in high doses, may cause copper deficiency, which can trigger anemia.

CR’s Take: The uncertain benefits of zinc aren’t worth the potentially serious risks. 

Editor’s Note: This article also appeared in the January 2018 issue of Consumer Reports magazine.