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A bottle of Z-Stack capsules


Z-Stack is a multivitamin marketed as an "all-in-one immune boosting super formula." The vitamin itself, which was developed by Dr. Vladimir Zelenko, MD — a physician known for developing the "Zelenko Protocol" for treating and preventing COVID-19 — does not appear to have been tested in any clinical trial, and there is only preliminary evidence that certain ingredients in Z-Stack may have immune-boosting effects, although not necessarily at the doses in Z-Stack. In fact, the amount of at least one of the ingredients in Z-Stack exceeds the Tolerable Upper Daily Intake Level, i.e., it raises the risk of adverse effects.

What ingredients are in Z-Stack and are they beneficial?
According to the product label, Z-Stack contains vitamin C, vitamin D3, zinc and quercetin. Since the exact ingredient combination in Z-Stack does not appear to have been tested in a clinical trial, we've reviewed the amounts of each of its ingredients for potential safety and immune benefit.

Vitamin C
Z-Stack contains 800 mg of vitamin C (as ascorbic acid) per daily serving. There is evidence that vitamin C may reduce the duration and severity of colds during cold season when taken in slightly higher doses (usually 1 gram daily) prior to getting sick. However, taking vitamin C does not seem to prevent colds unless you're deficient. Vitamin C supplements have been promoted by some websites to help fight viral infections other than colds, such as COVID-19. However, there is no evidence that getting more than the daily requirement of vitamin C (75 to 120 mg daily) can prevent infection from coronavirus, and clinical studies have not shown a clear benefit from giving high-dose vitamin C to COVID patients.

In general, vitamin C is safe when used in moderate amounts, although it may cause diarrhea, heartburn and other GI disturbances. However, the Tolerable Upper Daily Intake Level for vitamin C in adults is 2,000 mg — and this includes intake from foods, beverages and supplements. For people already getting a significant amount of vitamin C from their diet or other supplements, taking Z-Stack may cause them to exceed this level, which may increase the risk of side effects including kidney stones and kidney damage.

Vitamin D
Z-Stack contains 125 mcg (5,000 IU) of vitamin D3 per serving. This is concerning as it is not only way above the adult daily requirement (600 to 800 IU), but also above the Tolerable Upper Daily Intake Level for vitamin D (which is 4,000 IU for adults). Above this level, the risk of harm increases, including the risk of hypercalcemia (too much calcium in the blood), which can cause constipation, headache, irritability, confusion, weakness, metallic taste, loss of appetite, painful calcium deposits and kidney failure.

Research suggests that vitamin D may help reduce the occurrence of colds and upper respiratory infections in people without adequate blood levels of vitamin D. However, vitamin D supplements are not beneficial for colds in people who already have adequate levels of vitamin D. Since low blood levels of vitamin D have been linked with a greater risk of infection and more severe symptoms and complications of COVID-19, there has been interest in vitamin D supplements to reduce these risks. Preliminary research suggested that supplementing with moderate- to high-dose vitamin D when vitamin D levels are low may improve prognosis in people hospitalized with COVID-19, but there appears to be an increased risk of COVID-19 in people with very high levels of vitamin D, as could be caused by the amount of vitamin D in Z-Stack.

Z-Stack provides 30 mg of zinc (from 130 mg of zinc sulfate) per serving. Taking zinc is not likely to provide immune benefit for most people unless they are deficient in zinc, which is more common in elderly people, vegetarians, and those taking certain medications. In such people, supplementing with zinc (e.g., 20 mg per day, which is similar to the dose in Z-Stack) may help increase immunity and reduce the risk of respiratory tract infections. Although zinc lozenges have been shown to reduce cold symptoms, effectiveness appears to require a sufficient amount of zinc (as zinc gluconate or zinc acetate) taken as a lozenge so it can dissolve in the throat, where it is believed to act directly — not as a capsule, as in Z-Stack.

Zinc supplements cause few side effects other than mild stomach upset or unpleasant taste. However, the dose of zinc in Z-Stack is close to the Tolerable Upper Daily Intake Level for zinc, which is 40 mg daily. Getting too much zinc can interfere with copper absorption, which might suppress the immune system and cause other serious side effects. For this reason, if you are taking more than 40 mg of zinc daily, it's generally recommended to also take some copper (about 1 to 3 mg daily), which Z-Stack does not contain.

Z-Stack provides 500 mg of quercetin per serving. Although there is evidence that quercetin may help reduce upper respiratory tract infections after exercise, it is unclear if quercetin helps reduce the risk of colds. It's also unclear if taking quercetin supplements can help prevent or treat COVID-19 or what dosage would be effective.

Side effects of quercetin are uncommon but may include mild nausea, stomach upset and/or headache. The GI side effects can generally be reduced by taking quercetin with food. Be aware that quercetin may affect the uptake or metabolism of various medications, including certain statins, estrogens, methotrexate, midazolam, thyroid hormone, ibuprofen, and others.

Z-Stack costs $55 for 60 capsules (i.e., $1.83 per the recommended two-capsule daily serving). In ConsumerLab's product reviews, one can get the same amounts of these ingredients for as little as 25 cents (5 cents for 5,000 IU of vitamin D, 6 cents for 800 mg of vitamin C as ascorbic acid, 3 cents for 40 mg of zinc, and 11 cents for 500 mg of quercetin). So, you would be paying $1.60 extra per serving for these ingredients in Z-Stack, which would wind up costing you about $48 more per month.

The Bottom Line:
There is no evidence that taking Z-Stack boosts immunity. Although vitamin C, one of the ingredients in Z-Stack, may help reduce the severity of colds and other respiratory infections, the dose in Z Stack is slightly lower than what has been shown to beneficial. Vitamin D and zinc in Z-Stack may be beneficial if you have low levels, but the amounts of these vitamins in Z-Stack is somewhat higher than what would be necessary to correct mild deficiency. There's no strong evidence that the quercetin in Z-Stack helps with immunity.

If you believe you need any of the ingredients in Z-Stack, it would be less expensive to purchase each individually, and you can choose a dose that is correct for your individual needs.

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