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Why is there an interest in using zinc for the common cold?

Since there is no definitive cure for the common cold , conventional treatments aim to alleviate its symptoms. These treatments range from the use of decongestants and antihistamines for a runny nose to expectorants and antitussives for coughing. By contrast, zinc is thought to target the cold viruses that cause the symptoms. Zinc lozenges are marketed over-the-counter without a prescription. They are sold as dietary supplements in groceries, health food stores, and pharmacies. The lozenges are marketed in two forms; zinc gluconate and zinc acetate. The element zinc is the active ingredient in both forms.

Initially introduced in the early 1980's, zinc lozenges are being used by increasing numbers of people when they have colds. With the arrival of the current winter season, the lozenges once again have attracted the public's attention. In addition, the public's interest has been heightened by a recent settlement between a maker of the lozenges and the Federal Trade Commission (FTC) regarding the claims for zinc as a treatment for the cold. Accordingly, the role of zinc lozenges as a cold remedy is the focus of this update, which is based on our current in- depth review of the medical literature.

What is zinc?

In 1974, the Food and Nutrition Board of the National Research Council of the National Academy of Sciences established zinc as an essential mineral in our diet and provided a recommended daily allowance (RDA) for zinc. The RDA implies that a certain daily amount of zinc in the diet- -12 milligrams for women and 15 milligrams for men-- is essential to maintain good nutrition. Required by over 300 enzymes for their normal biological functions, zinc also is needed for the action of insulin in the body.

Good food sources of zinc include meats, liver, seafood (especially oysters), and eggs. Zinc also is found in whole-grain products such as whole wheat or rye bread, oatmeal, and whole corn. However, its effectiveness (or bioavailability) is less in these food sources. Bioavailability refers to the absorption of an element from the intestine into the blood stream so that it is available to participate in necessary biological functions in the tissues of the body. The type of food eaten can affect the bioavailability of zinc. For example, phytate (which is found in certain cereal proteins) and dietary fiber can reduce the absorption of zinc, and thereby decrease the bioavailability of zinc. An additional source of this essential mineral is multivitamin pills, which usually contain 100% of the RDA for zinc.

How do zinc lozenges work?

Zinc lozenges may work in several possible ways. For example, zinc might prevent the formation of proteins that are needed by a cold virus to reproduce itself and increase its own numbers. Without duplication of the virus, the cold symptoms will cease. Zinc also may attach to proteins that are located on the edge of a cold virus. This attachment impairs the ability of the virus to enter the body's cells, notably those in the respiratory system (the nose, throat, and lungs). Finally, in an undefined manner, zinc salts may protect and stabilize the lining of the cells, which thereby reduces the chance that the virus will penetrate the cells.

What is the evidence regarding the use of zinc lozenges for the treatment of the common cold?

Since 1984, twelve controlled studies (referred to as double-blind, placebo-controlled, randomized clinical trials) were conducted to evaluate the usefulness of zinc in treating the common cold. During a double-blind, randomized trial, which is considered by scientists as the most objective and reliable study method, neither the physician nor the patient knows whether the patient is being treated with zinc or a placebo. Which patients receive the zinc or the placebo is assigned in a random order (like the flip of a coin). The placebo in these studies is an inactive, non-zinc lozenge that is made to look like, and in some instances, taste like the zinc lozenge.

Five of the randomized clinical trials, which were done in adults, determined that zinc was effective in treating the common cold if the lozenges were started within 24 hours of the onset of cold symptoms. In one study, the lozenges reduced the severity of symptoms (assessed by using a scoring system). In the other four studies, the duration of symptoms was decreased by 3 to 4 days. In all of these trials, the lozenges contained 13.3 to 23.7 mg of elemental zinc and the patients took the lozenges once every 2 hours while awake.

In another study, the treatments with zinc or the placebo were begun before the volunteer subjects were purposely injected with a cold virus. The treatments were also continued afterwards. This is the only study to evaluate the effectiveness of zinc lozenges in preventing colds. The subjects in the zinc and placebo groups came down with about the same number of colds. The zinc lozenges therefore failed to prevent colds. However, and as experienced in the other trials, the severity of cold symptoms was significantly less in the group taking the zinc than in those taking the placebo.

In five other randomized, controlled trials in adults, there was no reported benefit of zinc in treating the common cold. A close examination of each of these trials, however, revealed one or more possible explanations for the lack of benefit. In some of these trials, the dose of zinc may have been too low. For example, in one trial, each lozenge contained only 4.5 mg of zinc. In four of these trials, compounds such as citric acid, tartaric acid, mannitol, and sorbitol, which were added to improve the flavor of the zinc lozenge, may have impaired the bioavailability of the zinc. These added compounds can form chemical bonds with zinc, thereby making the ingested zinc unavailable for its therapeutic effect.

The only study done in children and adolescents showed no value of zinc lozenges in treating the common cold. This was a large, randomized, controlled trial that was well-executed. The conclusion, therefore, is that either the 10 mg of elemental zinc used in this study may have been too low to relieve cold symptoms in children or the beneficial effects of zinc on the severity and duration of colds are limited to adults.

What are side effects of zinc lozenges?

Zinc lozenges have two noteworthy short-term adverse effects-an unpleasant taste and nausea. Side effects of zinc lozenges taken over a longer period (for example, in trying to prevent colds) have not yet been studied. Theoretically, however, an excess of zinc in the diet could interfere with the absorption of copper, which then could become deficient in the body. Accordingly, zinc lozenges should be limited to short-term therapy, such as the treatment of cold symptoms for up to about a week.

As for using higher doses of zinc, taking 300 mg of elemental zinc daily for six weeks impairs the body's immune responses. This would theoretically increase a person's susceptibility to infections. To consume that much elemental zinc daily from zinc lozenges alone, a person would have to take more than 22 lozenges per day (each containing 13.3 mg), which is almost double the maximum recommended dose. High doses of zinc also are associated with decreased levels of high-density lipoprotein cholesterol (the "good" cholesterol) in the blood, which could be another undesirable effect.

Adverse interactions between zinc lozenges and other drugs have not been reported to date. However, citric acid, which is found in oranges, grapefruits, and lemons, can inactivate zinc salts. While vitamin C has been reported to increase the absorption of iron, a similar effect on zinc has not been seen.

How do I take zinc lozenges in the treatment of colds?

Zinc lozenges are available in the form of zinc acetate and zinc gluconate, with instructions for use on the label. Zinc acetate lozenges have not been studied as extensively as zinc gluconate, so it is uncertain whether or not zinc acetate is of equivalent value. Actually, zinc acetate has a higher bioavailability of zinc, which has given this formulation an added appeal. Although zinc acetate lozenges have a patent from the U.S. Patent and Trademark Office, the issuance of a patent, even with the title of Cure for Common Cold, does not guarantee clinical value. Therefore, zinc gluconate is preferable for now.

Most products contain 13.3 mg of elemental zinc in each lozenge. Depending on the brand purchased, instructions are to dissolve one lozenge slowly in the mouth every 2-4 waking hours, starting as soon as symptoms are noticed. The consumer is instructed not to exceed 6-12 lozenges per day. Citrus fruits and juices and probably dietary fiber, which can inactivate or decrease the absorption and bioavailability of zinc salts, should be avoided an hour before and after sucking a lozenge.

What controversy has there been about the usefulness of zinc for colds?

The Quigley Corporation, based in Doylestown, Pennsylvania, manufactures the popular over-the-counter zinc gluconate lozenges named Cold-Eeze and Cold-Eeze Plus. On November 23, 1999, the company agreed to settle with the FTC on federal charges of unsubstantiated clinical claims that were made several years ago. Under the settlement, Quigley can no longer claim that the lozenges prevent colds, alleviate allergy symptoms, or decrease the risk of pneumonia . However, this settlement left intact Quigley's central claim that Cold-Eeze can decrease the severity and shorten the duration of cold symptoms in adults.

Should I take zinc lozenges when I have a cold?

A considerable amount of scientific data argues that zinc gluconate lozenges reduce the duration of cold symptoms in adults. Still, not all authorities agree that zinc lozenges are effective. After our in-depth review of the existing clinical trials, however, we conclude that zinc is effective in shortening the duration and possibly decreasing the severity of cold symptoms in adults -- which is nothing to sneeze at. Remember, however, that currently this treatment is not for children with colds. Additionally, taking zinc while healthy in order to prevent a cold is not advised.

Remember also that the effectiveness of zinc lozenges relies on frequent dosing for the full length of the illness. Therefore, adult cold sufferers and their physicians will have to decide if the benefit of a shortened cold outweighs the risk of the adverse effects and the inconvenience that comes with such frequent dosing.

What about future use of zinc lozenges for colds?

In the future, there will probably be yet more clinical trials evaluating zinc for colds. These trials are needed to determine: (a) the value of zinc acetate as compared to the better-studied zinc gluconate in treating the common cold, and more importantly, (b) whether zinc, perhaps in some new formula or schedule in taking the mineral, can be effective in treating colds in children or in preventing colds in adults and children. Data also will be obtained on the adverse effects of the long-term use of zinc. Meanwhile, investigators will try to improve the taste of the zinc without reducing its effectiveness. Finally, trials in the future should include viral studies to support the clinical findings and to determine just how zinc works in affecting cold viruses.

Zinc Lozenges At A Glance

• Zinc lozenges shorten the duration of symptoms from the common cold, such as a cough and runny nose, in adults but not in children.
• The lozenges must be taken frequently, starting as soon as symptoms are noticed (within 24 hours of the onset).
• Adverse side effects of short-term use of zinc are an unpleasant taste and nausea: for longer use, side effects are unknown.
• Patients and their physicians can decide whether the benefit of a shorter cold outweighs the inconvenience of frequent doses and the unpleasant taste of the lozenges.
• No preventive treatment or ultimate cure for the common cold is available.

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