In the past few weeks, Ashley Koff, a registered dietitian in Columbus, Ohio, has gotten a deluge of requests from people who want to know which dietary supplements they should take to protect themselves from the coronavirus.

People ask how much elderberry syrup, colloidal silver and oil of oregano they should take to boost their immune systems. One person who tested positive for the coronavirus and was sent home from a hospital to recover texted Ms. Koff asking for supplement recommendations. Others wonder whether they should drink ample amounts of kombucha and use essential oils as hand sanitizer.

“Someone asked me if they should do a detox juice cleanse and I told them absolutely not,” said Ms. Koff, who is the chief executive of The Better Nutrition Program, which provides nutrition advice to individuals and corporate wellness programs. “Someone else asked me if they should be doing a fast. I told them anything that’s depleting your body of resources right now is going to increase the risk of you getting sick.”

Dietary supplement sales have surged nationwide as panicked consumers stock up on vitamins, herbs, extracts, and cold and flu remedies. None of these products have been shown to lower the likelihood of contracting the coronavirus or shortening its course, and taking large doses of them can potentially do harm. But experts say that the jump in sales suggests many people are desperate to strengthen their body’s immune defenses and ease their heightened anxiety levels.

“The numbers we are seeing are unprecedented,” said Joan Driggs, an analyst at IRI, a market research firm that tracks supplement sales at Walmart, Walgreens, Safeway, CVS and other drugstores and retailers. “People are trying to protect themselves, and they feel that they have to take advantage of whatever resources are at hand.”

While sales of dietary supplements rose 6 percent over all during the first week of March compared to the same time period last year, according to IRI, sales of cold and flu remedies surged. Sales of vitamin C, a purported immune booster, soared 146 percent, while sales of zinc, a popular remedy for colds and respiratory illness, shot up 255 percent. Sales of elderberry supplements, which are marketed for immune support, are up 415 percent. Echinacea, an herb used for cold and flu relief, jumped 122 percent.

Vitamin D and multivitamins for children and adults have also experienced unusually sharp spikes in sales this month. So has melatonin, which some studies have shown may aid sleep. Ms. Driggs said that the growth in melatonin was probably related to an increase in stress-related sleep disruptions. “It signals to me that there are higher levels of anxiety,” she added. “People are frightened for their family’s health and well-being and also their financial well-being.”

Across the country, pharmacies dealing with dwindling supplies have placed limits on the amounts of vitamins, Zicam, Emergen-C and other supplements that customers can purchase. One large supermarket chain with 230 stores in seven states, Harris Teeter, restricted sales of Airborne, which is marketed for immune support, to three packages per customer.

The frenzied buying has led some companies and wellness enthusiasts to prey on public fears. Countless natural remedies for coronavirus have circulated on social media, including herbs, saltwater, creams, toothpastes and “miracle mineral solution,” a mixture of sodium chlorite that the Food and Drug Administration has warned is tantamount to drinking bleach.

In response, the F.D.A. and Federal Trade Commission sent warning letters to at least seven companies warning them to stop advertising fraudulent cures and treatments for coronavirus, including essential oils, elderberry teas and colloidal silver supplements. The agencies cautioned that colloidal silver in particular is “not safe or effective” for any disease or condition. They said they were monitoring websites and social media to root out bogus products and that a task force has worked with major retailers and online marketplaces to remove dozens of listings for fraudulent coronavirus cures and treatments.

Americans already have a high level of anxiety about the spread of coronavirus, said Joseph Simons, the chairman of the F.T.C. “What we don’t need in this situation are companies preying on consumers by promoting products with fraudulent prevention and treatment claims,” he added.

Five of the major supplement industry trade groups applauded the government action and urged consumers to be wary of products making coronavirus-related claims. The industry has also lobbied the federal government and state and local officials to classify dietary supplement makers as “essential” in areas where there are shelter-in-place ordinances restricting nonessential commerce.

“I think the fact that we’re seeing this spike in sales tells us that consumers absolutely value their ability to use supplements,” said Steve Mister, the president and chief executive of the Council for Responsible Nutrition, an industry group.

The fastest selling supplements are those that have long been the subject of research on cold and flu relief, including zinc, vitamin D and elderberry extract. Zinc is thought to inhibit replication of the virus that causes the common cold. Some randomized trials have found that taking high doses of it may help to lower the risk of contracting a cold and potentially shorten its duration by 20 percent.

Supplementing with moderate doses of vitamin D has been shown in some trials to help lower the risk of contracting the cold and flu, but the effect is mainly seen in people who have very low or deficient levels. And a handful of small, industry-funded trials have found that elderberry extract can shorten the duration and severity of the cold and flu.

But the evidence for most of these supplements is not strong, and it’s not possible to extrapolate that they will help prevent or treat coronavirus because they have some efficacy against the cold and flu, said Kamal Patel, a nutrition researcher and the director of Examine.com, a large and independent database of supplement research. These viruses are quite different from one another. The coronavirus, for example, has a more severe impact on the lower respiratory tract and a longer incubation period compared to the viruses that cause colds and influenza.

Taking large doses of single vitamins and minerals also carries risks. Excessive levels of zinc, for example, can disrupt the body’s uptake of copper, increasing the likelihood of anemia. Vitamin D is not metabolized efficiently without an adequate level of magnesium, and in high doses it can be toxic.

Vitamins and herbal supplements can also interact with prescription medications, dampening their effectiveness or, in the case of blood thinners, for example, raising concentrations to dangerously high levels.

There are times when taking a supplement can be very useful, such as during pregnancy or to address a clear nutrient deficiency. But for healthy adults who are worried about the coronavirus, eating a nutritious diet and getting proper sleep and exercise are the best ways to strengthen your immune system, said Linda Van Horn, chief of nutrition in the department of preventive medicine at the Northwestern University Feinberg School of Medicine.

Whole foods like fruits, vegetables, fish, poultry, nuts, legumes and milk contain a wide range of vitamins, minerals and phytochemicals — including zinc and vitamin D — that work in synergy to protect your health.

“This is an ideal time to look at what you’re eating,” said Dr. Van Horn. “We all know that grocery stores have been experiencing some limitations. But for the most part people are still able to find fresh produce and other healthy foods.”

Ms. Koff, the dietitian in Ohio, said she tells people it is fine to take a multivitamin to address any gaps in their nutrition. But she encourages people to focus on their diet, stress levels and sleep and warns them not to overload their systems with large doses of supplements.

“This is the time to start implementing behaviors that support your health, not going and taking high amounts of things that are incorrectly listed as immune boosters,” she said.