Support immunity

The mechanism is shown far more directly in the animal studies than in the human ones, where nobody has followed it over time. And this is about how cells respond rather than how much hormone is present, so a blood cortisol reading says nothing about whether it is happening.

For the other oral decongestants pooled in that search, a week or more of daily use changed systolic pressure by 0.91 mmHg on average, at very low certainty. None of those trials used phenylephrine, so the figure cannot be carried over to it.

The trial's own title calls it open-label, so people may have known what they were taking. That usually inflates a self-rated benefit rather than hiding one. Everyone in it had hay fever, not a cold.

All four trials were in hay fever, so none tested a blocked nose from a cold. Two of them gave a single dose and followed congestion for 6 hours. The largest used a 30 mg slow-release tablet, not the 10 mg tablet sold in cold remedies.

Only five comparisons split the symptoms this way, out of ten trials in the pooling. Severity was measured inconsistently, in some trials by counting days spent indoors, which mixes how ill someone felt with whether they could take time off work.

A guideline grades other people's evidence rather than producing any, and most of its positions are worded as suggestions, which is how low certainty gets expressed. It sets aside anyone with an established medical reason to be tested or treated, so it speaks only about people without one. Two corrections to it were published in 2025 and its paediatric advice has drawn published rebuttals.

Each of these trials recruited a different population and measured a different outcome, so what they share is a pattern rather than one replicated result. Three of the four used large intermittent doses, a schedule some analyses argue is the least effective. In one of them, the group whose levels reached the recommended target fell more often than the group whose levels did not.

The outcome in both studies is a blood level; neither measured whether anyone felt or functioned better. The 87 percent figure comes from 33 people on a single-blind protocol at a weekly dose, which makes it a statement about that regimen rather than about a daily D3 tablet against a daily D2 one. The pooled comparison searched the literature only to July 2011.

The height gain is a standardised effect of 0.12, which is real across a population and invisible in any one child. The diarrhoea figure came from 19,468 children while the mortality analyses carried over 130,000 each, which is part of why the nulls in this review are the more certain results. The trials ran mostly in low- and middle-income countries where zinc shortage is common.

The children were mostly under five, in low- and middle-income countries, on 10 to 15 mg of zinc a day for a median of 26 weeks, so this is not evidence about supplementing a well-fed child anywhere else. The review's own summary describes zinc as probably reducing deaths from chest infection and from malaria, but both of those ranges cross into no effect and the authors add that they could not rule out an increased risk of death. A null on mortality is not a null on everything the same trials measured.

Nine trials and 1,449 people, graded low certainty. The review pooled lozenges, syrups, tablets and nasal gels as one treatment, with the gluconate lozenge doses alone spanning 45 to 276 mg a day. It also excluded any product mixing zinc with other vitamins or herbs, which is most of what a pharmacy actually sells.

The figure comes from roughly a week of high-dose lozenges taken during a cold, not from a daily supplement taken for months. It counts non-serious events only, and no treatment study recorded a serious one. Sixteen trials contributed, most at unclear or high risk of bias in at least one area.

Most of these trials ran in Asian countries where zinc shortage is common, and the review's authors say plainly that the evidence does not support giving zinc to well-nourished children or in low-risk settings. Vomiting rose in both age groups, by about 57% in the over-sixes, which is a real cost in a child already losing fluid. Death and hospitalisation, the outcomes that matter most, sit at very low certainty and are not settled here.

Neither finding comes from a trial designed to test the pairing: one is a subgroup pattern and the other a set of marker outcomes graded very low to moderate certainty. The usual explanation, that two similar metals compete for the same route into the body, is not tested by either source. All of it concerns children given substantial doses of both minerals in settings where deficiency is common, not the small amounts in a multivitamin.

This is a narrative review drawing human and animal work together, not new data. And the arrow could run either way: a stress-disrupted cortisol system might raise autoimmune risk, or years of autoimmune illness might be what disrupts the system. Nothing here separates the two.

The outcome is a marker in spit, not a symptom: nothing was reported about breathlessness, flare-ups, lung function or quality of life. The confidence interval runs from a 76 percent reduction to a 7.6 percent one, so the size of the effect is barely pinned down. Inflammation in the blood did not change, so the effect did not travel beyond the airway. Two authors are employees and shareholders of the company selling the compound.

The carvacrol in the mouse study came from a different plant, and the mice were infected through the nose, far from a person swallowing a capsule. A dish holds the oil against the germ directly, and these studies do not show whether a swallowed dose reaches an infection at that strength.

The US trial registry lists 19 studies of oregano and none for a cold, so no cold trial is registered either. The one oregano trial that did turn up was published outside the main medical indexes, so a small trial published the same way could have been missed.

The labelled amounts are the makers' own and none states a standard carvacrol content. Only combination products with black seed oil were checked, so plain oregano softgels were not compared.

The oregano in it is Origanum syriacum, not the Origanum vulgare sold as oil of oregano. It was blended with mint, eucalyptus and rosemary oils, so no one oil's effect can be picked out. The spray went on the throat and was not swallowed.

More phenylephrine in the blood is not shown to mean a clearer nose: no study tested whether the pairing relieves congestion better. The studies gave single doses to healthy volunteers. Their authors work for a company that makes combination painkillers.

The review's young-children finding covers cold remedies with mixed ingredients, most of them other decongestants, so it is not phenylephrine's own result. The children absorbed the drug much as adults do, with somewhat lower peak levels.

The one modern placebo trial begun in real colds was stopped early for low enrolment and never published in a journal. Older single-dose laboratory studies, from the 1950s to 1970s, were never published either.

A label lists what the maker is permitted to claim under US rules, not proof that the claim holds. Labels change, and this one took effect in June 2026.

An advisory vote is advice to the FDA, not a ruling; the agency decides separately. Members noted that no data measure the drug in the nasal lining.

The status is dated: it reflects the FDA's order page on 26 September 2026. The proposal concerns effectiveness, not safety. It covers phenylephrine taken by mouth, not nasal sprays.

No study has measured phenylephrine in the lining of the nose, where it would have to act. So the link between little drug in the blood and no relief is inferred, not shown.

These are reports of individual cases gathered in a narrative review, so they show that it can happen, not how often.

Both studies gave single doses, so neither shows what a day or a week of doses does. Everyone in them had normal blood pressure. The larger study was run by employees of a phenylephrine maker. A model built from eye-drop data predicts a larger rise when 10 mg is taken with 1,000 mg of acetaminophen, 12.3 against 4.2 mmHg in average arterial pressure, a prediction nobody has measured.

Each person took all three in turn, and the authors flag that earlier doses may have carried over: in the first round alone, phenylephrine's gap over placebo was 64% of pseudoephedrine's, against 17% across all rounds. It tested a single dose. The congestion came from pollen, not a cold.

This curve was mapped in seven volunteers living in hospital on a near-zero vitamin C diet for four to six months, which buys precision at the cost of a very small and unusual sample. It describes concentrations in blood and cells, not illnesses avoided. The work is thirty years old.

The trials pooled here run from the 1960s onward, defined a cold in different ways, and mostly counted them by asking people. The two groups that agree worked from largely the same trial set, so this is one body of evidence read twice rather than two independent verdicts.

Nobody counted a stone here, and three days is nowhere near long enough to. Everyone took the vitamin and everyone was measured before and after, with no placebo group to compare against. The 2 gram group rose less than the 1 gram group, so dose and excretion do not line up cleanly.

These are standing pharmacological facts from a clinical reference chapter rather than findings from a trial. The chapter does not say how long supplementation takes to raise a level, or the dose at which toxicity begins. Nothing in it settles what level to aim for, which is a separate and open question.

This is a snapshot of two existing groups at one research unit, so it shows which mathematical curve fits best rather than testing the mechanism directly. Ruling out fat trapping the vitamin rests on that better fit, which is suggestive rather than conclusive. The advice to scale a dose by body weight follows from the model and was not itself tested here.

That upper limit was set on the risk of high blood calcium with an allowance for uncertainty, not on any observed pattern of harm at 4,000 international units. Reaching a toxic blood level takes very large doses sustained over a long period. Safety at 2,000 international units daily says nothing about the very large intermittent doses that were followed by more falls in other trials.

The notice is careful about what it did and did not establish: the duplicated text was judged minor and correctable, and the data allegation was investigated and left inconclusive, which is not a finding of misconduct. The complainant is himself the author of the competing review that the withdrawal left standing. The withdrawal covers the 2013 version specifically.

These are null results in very small groups: three acetate trials against four gluconate ones, and five lower-dose trials against two higher-dose. Failing to show a difference in 575 people is not the same as showing there is none, and the ranges are wide enough to hide a real one in either direction. Every trial used lozenges in adults with ordinary colds, so none of it reaches children, prevention, or any other route.

This comes from a narrative review written as a call to action, so it argues a position rather than weighing evidence neutrally, with no stated search strategy or bias assessment. Two of its authors work for an organisation promoting biofortification, one of the solutions the paper recommends. Its own link between zinc and blood sugar or heart risk is described as growing evidence rather than established, and should not be read as settled.

Every outcome here is a laboratory marker rather than an illness, so this shows zinc moving numbers, not children coming to harm. The authors say outright that whether the findings matter clinically is unclear, and the certainty runs from very low to moderate. Each adverse finding held in at least one subgroup rather than across the board, so none of it describes a general effect of a stated dose.

A label is a regulatory and marketing document, not evidence: the pack's stated purpose of reducing cold duration is a manufacturer claim made under the homeopathic route, not one a regulator reviewed. Nothing here says the product does or does not reach the dose the trials used, only that the label does not let you check. This is the oral tablet, a different product from the nasal gels that drew the smell-loss reports.

Five small trials and 598 people between them, in settings almost nobody lives in: a subarctic military exercise, a 90 kilometre race, an Alpine ski camp. None of them tested ordinary gym-goers. The underlying trials date from the 1960s to the early 1990s and counted colds largely by asking people.

Earlier versions of this review called half a day of doubtful clinical usefulness. The pooled trials span six decades, with doses from 0.2 grams a day upward and no shared definition of a cold. One researcher has authored the review across its updates and argued this finding publicly for years.

Whether daily vitamin C shortens colds turns on which evidence counts. Every trial showing a benefit dosed people before the cold began, and a stricter overview of 8 primary studies found little or no effect.

A subgroup result is a comparison the trials were never designed to make, which is why it is the kind of finding that most often fails to repeat. Those 255 deficient people produced an odds ratio of 0.60, an interval running from a 72 percent reduction to a 30 percent increase. Only 2.4 percent of that trial's participants were deficient at all.

Whether vitamin D protects severely deficient people from respiratory infection was never properly tested. A 2017 estimate of 70 percent lower odds did not reappear in larger analyses, and trials enrol too few deficient people to tell.

The 2025 funnel plot leans left, the pattern seen when small trials that found nothing were never published. All three versions pool trials running from infancy to age 95 across many countries, so the pooled group is not one population. The 2017 version worked from individual participant records and the later two from summary numbers, which is a weaker footing for any subgroup.

Protection against respiratory infection shrank each time the same analysis was rerun with more trials. Whether the effect was never there, or the newer trials enrolled people with less to gain, these analyses cannot separate.

The p value lands exactly on 0.05 and the interval nearly touches no effect, which makes this a fragile positive rather than a settled one. The count pools rheumatoid arthritis, polymyalgia rheumatica, autoimmune thyroid disease and psoriasis together and tests none of them individually. Participants averaged 67 years old.

The oregano nasal-spray trial ran at one clinic with 25 people a group and has not been repeated. Every group also followed the same Persian-medicine lifestyle routine, which may explain part of the placebo group's gain. A 2023 pooled ranking that put oregano first for this condition rests on this same trial.

Everyone in the supplement trial took the vitamin first and the placebo second, so an order effect cannot be separated from the treatment. Glutathione inside a lymphocyte is a laboratory reading, not an illness avoided. One author of the mechanism review is employed by a vitamin C manufacturer.

People who take a gram of vitamin C a day may differ from those who do not in ways a diet questionnaire misses, so this is a pattern rather than a cause. Hardly anyone ate more than 700 milligrams from food, which leaves the food comparison untested at the doses where the supplement finding appears.

The strongest single trial here lost 251 of its 400 volunteers before any diary came back, and how ill someone felt could plausibly decide whether they filled one in. Its comparison group still took 0.03 grams of vitamin C rather than nothing. None of these trials went past 3 grams a day.

Vitamin C started at the first symptom failed in seven trials, all stopping at 3 to 4 grams a day. Two older trials hint an effect only emerges at 6 to 8 grams, and that dose has never been tested directly.

This is a snapshot of blood levels taken in 2005 and 2006, so it shows who sits below a line and never what being below it does to them. Race here stands in for skin pigmentation, sun exposure, diet and income all at once, rather than for anything measured biologically. Black Americans measure lowest without correspondingly worse bone density or fracture rates, which is the part the number does not explain.

The 20 nanogram figure was set as a population planning number covering the needs of nearly everyone, not as a line for diagnosing one person, and the committee that set it has said so in print. The Institute of Medicine derived it from bone health alone, having found the evidence on cancer, heart disease, diabetes and infection too thin to set requirements on. Which number a person is measured against decides whether they are told they have a problem.

Where vitamin D deficiency begins is unsettled: expert lines sit at 30, 20 and 12 nanograms per millilitre. Evidence cannot choose between them, because no trial has tested whether measuring a level and acting on it does anyone good.

Fifty people in the primary trial, which reported colds 44% shorter, far above anything the wider literature supports. That trial is one of the three inside the pooled analysis, so the two are not independent confirmations of each other. The finding that the effect held regardless of allergy, smoking, age or gender rests on those same 199 people, too few for that comparison to carry weight.

Eight trials, 972 people. The estimate mixes zinc acetate, gluconate and orotate given as lozenges, syrups and tablets, so it is an average over treatments that may not behave alike. Most of the included studies carried an unclear or high risk of bias in at least one area.

Zinc lozenge trials scatter widely. One reading says that makes any effect size untrustworthy; another sees a dose split, nothing below about 75 mg a day and a great deal above. No trial has split them, so how much zinc helps is open.

Thirty-seven cases gathered over 53 years have no denominator, so nothing here says how likely this is at any dose, and case reports get published because they are striking. The zinc came from denture adhesive cream and swallowed coins as well as supplements, so not every dose in that range is a supplement dose. A single case at 50 mg a day is not evidence that 50 mg a day is generally unsafe.

Adverse-event reports are voluntary and unverified, so 130 counts reports received rather than confirmed cases, and it cannot be turned into a risk per user in either direction. The causality analysis is a structured argument rather than a controlled study, and it comes from the same clinic and senior author as the case series it draws on. All of it concerns the nasal gel: lozenges and tablets are a different product taken a different way, with no comparable evidence of smell loss.

The 75 mg line was drawn after looking at the data rather than set in advance, so it explains the same trials it came from. Five trials below it and eight above is a thin basis for a threshold. Both reviews behind it have the same single author, whose complaint is what got the competing Cochrane review withdrawn.

An upper limit is a population ceiling for long-term intake from all sources, not the dose at which harm begins in a person. The short lozenge regimens that shortened colds ran at 80 to 92 mg of zinc a day and exceeded it deliberately, for about a week, which is not an error in either the trials or the limit. The US figure was set in 2001 and has not been revised for zinc since.

Europe caps zinc at 25 mg a day and the US at 40, from the same copper evidence. No new data separates them, only a judgement on how much copper loss is acceptable, and nothing settles which reading is right.

Oregano was one oil among several, so its own share cannot be told apart. The study was a pilot comparing three blends. No study of oregano taken with metformin or another diabetes drug has reported results.

An untested pairing is not a safe one; a bleeding effect has not been measured either way. A US trial of how oregano changes the body's handling of a set of drugs has been recruiting since 2024 and has posted no results.

The birds ate oregano as 2% of their feed without a break, far more and for far longer than a person on capsules. A change in gene activity is not an illness, and no illness was linked to it in the chickens.

It has never been tried in an animal or a person. The strengths used, 58 to 588 µg per mL, have not been shown to be reachable in the human gut from a supplement.

In the only trial of oregano oil for gut parasites, nobody went untreated, and parasites can come and go in stool on their own, so their clearing may not be the oil's doing. Two of the three parasites were found in 4 people and 1 person. No trial with a placebo group has followed in the 25 years since, and none is registered.

Case reports cannot show that the decongestant caused the colitis. Unusual cases are the ones that get written up and published. With 3 phenylephrine cases, no rate can be worked out.