NOTE: THESE SUMMARIES ARE OF HISTORICAL INTEREST ONLY AS THE COCHRANE REVIEWS ARE OUTDATED
- Echinacea: 16 trials
- Vitamin C in high doses: 56 trials
A cold is a common sickness caused by viruses that can lead to a runny nose and congestion, inflamed sinuses and headache, sore throat, sneezing, chills and fever. These symptoms can be described as common cold, influenza-like syndrome or in medical terms as an unspecific viral upper respiratory tract infection. Because the common cold is usually caused by respiratory viruses, antibiotics do not help. Symptoms may be severe enough to confined people indoors or for them to take time off work or school.
We present some of the evidence from Cochrane systematic reviews about complementary and supplemental therapies for the common cold. This evidence comes from carefully researched reviews of information about clinical trials done to evaluate healthcare treatments. Studies are only included in these reviews if they meet pre-defined criteria.
ECHINACEA FOR PREVENTING AND TREATING COMMON COLDS
Preparations of the plant Echinacea (family Compositae) are widely used in some European countries and in North America for preventing and treating common colds and increasing the body’s defences (immunity). The preparations or products available on the market differ as they use different types (species) and parts (herb, root, flower) of the plant and are prepared using different manufacturing methods (drying, alcoholic extraction or pressing out the juice from fresh plants). Sometimes other herbs or homeopathics are also added.
The three main species in medical use are: Echinacea purpurea (E. purpurea), Echinacea pallida (E. pallida), and Echinacea angustifolia (E. angustifolia).
What the synthesised research says
Preparations made up of alcoholic extracts of Echinacea or pressed juice from the aerial parts of Echinacea purpurea might have beneficial effects on cold symptoms (including runny nose) in adults if treatment is started early. Not all trials using such preparations showed even a trend for an effect, however. Beneficial effects in adults of other Echinacea preparations or for preventing colds might exist but have not been shown in independently replicated, rigorous randomised trials. There is no clear evidence that children benefit.
The majority of trials investigated whether taking Echinacea preparations after onset of cold symptoms shortens duration or decreases severity of symptoms, compared with a non-active placebo.
Two trials (three comparisons) investigated whether taking Echinacea preparations for 8 to 12 weeks prevented colds but found no clear effect compared with placebo.
How it was tested
Sixteen randomised controlled trials including a total of 22 comparisons of an Echinacea preparation and a control group (19 with placebo, 2 with no treatment, 1 with another herbal preparation) met the inclusion criteria for the review. All trials except one were described as double-blind and the majority were of reasonable to good methodological quality.
Comparing an Echinacea preparation with placebo as treatment for a cold acquired in the community, a significant effect was reported in nine comparisons, a trend in one, and no difference in six. More than one trial was available only for preparations based on the aerial parts from Echinacea purpurea.
None of the three comparisons of Echinacea preparations and placebo in the two prevention trials showed clear differences in the number of participants with and without active treatment on one or more cold episodes, duration, or severity of colds. In all three comparisons the number of participants with at least one cold was slightly lower with the tested Echinacea preparations than with placebo.
Side effects and general cautions
The Echinacea preparations tested in the clinical trials differ greatly. In these trials, side effects or adverse events associated with Echinacea preparations were infrequent and minor and were generally to those reported by people taking placebo. Rashes were reported in one trial involving children; another reported minor gastrointestinal symptoms and headaches or dizziness in adults.
As randomized trials include limited numbers of participants and often exclude persons with other illnesses, a review of such trials can only contribute limited knowledge on safety issues. The most relevant potential adverse effect of Echinacea preparations are reported to be allergic reactions.
Studies on individuals suffering from other upper respiratory tract infections with a defined cause, for example influenza, acute sinusitis or tonsillitis, were not included in this review.
Trials on combinations of Echinacea and other herbs were not included in this review.
Linde K, Barrett B, Wölkart K, Bauer R, Melchart D. Echinacea for preventing and treating the common cold. Cochrane Database of Systematic Reviews 2006, Issue 1. Art. No.: CD000530. DOI: 10.1002/14651858.CD000530.pub2.
Review authors’ noted: “We are somewhat concerned by the fact that results of placebo-controlled treatment and self-treatment trials were either clearly positive (significant effect of the tested Echinacea preparation over placebo) or negative (no or very minor differences) while only one trial had a less clear result. We would have expected a larger number of trials showing less extreme results. One possible reason for our finding might be reporting bias; that some authors tended to report only those findings which were statistically significant.” Another reason might be that people did not stay blinded to who was receiving the active treatment in some of the positive trials.
VITAMIN C FOR PREVENTING AND TREATING COMMON COLDS
People may not have sufficient intake of vitamin C in their diet for optimal health. Additionally, taking large doses (mega-dose supplementation) might reduce both the number of colds and their severity and duration, by improving respiratory defence mechanisms.
The role of oral vitamin C (ascorbic acid) in the prevention and treatment of the common cold has been a subject of controversy for at least sixty years. Vitamin C continues to be widely sold and used for prevention of colds and treatment of cold symptoms.
This review of evidence looked at comparisons of orally administered vitamin C of at least 200 mg daily and a placebo (which in a few instances included a low dose of vitamin C (up to 70 mg/day) to ensure that participants were not vitamin C deficient as regular dietary intake of vitamin C varies a lot.
To explore the role of vitamin C dosage, each study comparison was sorted using the dose of vitamin C: (1) more than or equal to 200 mg and less than 1 g vitamin C per day; (2) more than or equal to 1 g and less than 2 g vitamin C; (3) more than 2 g.
What the synthesised research says
Taking vitamin C supplementation did not reduce the incidence of colds in the normal population, indicating that routine mega-doses are not rationally justified for community use.
Evidence shows that vitamin C could be justified for people exposed to brief periods of severe physical exercise or cold environments and could halve the risk of getting a cold.
Taking vitamin C regularly had consistent small benefits on duration and severity of colds.
The trials in which vitamin C was given at the onset of colds did not show any benefit in doses up to 4 grams daily. One large trial reported equivocal benefit from an 8 gram therapeutic dose given at onset of symptoms; two trials using a longer five-day supplementation also reported benefits.
Preventing a cold (prophylaxis)
Thirty trial comparisons involving 11,350 study participants contributed to the data on the relative risk of developing a cold while taking vitamin C or a non-active placebo for two weeks to six months. The risk of a cold was not different with vitamin C or placebo. A subgroup of six trials that involved a total of 642 marathon runners, school children in a skiing school, and soldiers on sub-arctic exercises reported a pooled relative risk (RR) of 0.5 (range 0.4 to 0.7). For three of them, the dose of vitamin C used as prophylaxis was less than 1 g daily so that the effect in this subgroup is not explained by taking the highest doses.
The duration of colds experienced while trial participants were taking vitamin C demonstrated a modest but consistent and statistically significant benefit to the vitamin C supplemented participants, more so for children than adults. Thirty comparisons that involved 9,676 respiratory episodes showed a consistent benefit on cold duration, representing a reduction of 8% (range 3% to 13%) for adults (22 comparisons) and 14% (range 5% to 21%) for children (12 comparisons). The practical relevance of these findings are open to question since the therapeutic trials carried out so far have not found overall benefits.
For severity of colds, 15 trial comparisons that involved 7,045 colds showed a small difference favouring those people on vitamin C when cold symptoms started - for days at home and off work or school. Eight studies showed a trend for reduced symptom severity scores.
The great majority of the trials used 1 g/day of vitamin C.
Starting to take treatment after cold symptoms begin
Seven trial comparisons (eleven arms) that involved 3,294 respiratory episodes looked at cold duration during therapy with vitamin C that was initiated after the onset of cold symptoms, and no consistent difference in duration was seen compared with people taking placebo.
Four trial comparisons (eight arms) that involved 2,753 respiratory episodes showed no clear effect of vitamin C on cold severity.
None of the therapeutic trials have examined the effect of vitamin C on children, although the effect of prophylaxis on duration has been substantially greater in children compared with adults, and children have substantially higher incidence of acute respiratory tract infections.
In three trials volunteers were intentionally exposed to known viruses after preliminary dosage with megadose vitamin C or placebo, to see if they would develop a cold or not. Different ways of exposing the vitamin C or placebo treated volunteers to the virus gave different results. Those infected by nasal installation of the virus received a small or no benefit from vitamin C, whereas a group who were infected more naturally, from fellow volunteers given the virus, reported less severe symptoms. Virus shedding was similar in both treatment groups ie both groups had the virus to the same degree.
How it was tested
Fifty-six trials contributed data to this review: three laboratory prophylaxis trials in which volunteers were intentionally exposed to known viruses; 42 community prophylaxis trials which evaluated the effects of daily supplementation with vitamin C on reducing the incidence or severity of naturally acquired colds; and 11 community therapeutic trials that evaluated treatment with high dosage vitamin C after natural common cold symptoms had commenced.
Side effects and general cautions
Trials differed in quality, vitamin C dosage, age of participants, and the particular life circumstances of the participants.
Most investigators have used self report by participants of a widely agreed constellation of symptoms and the self-assessed duration and severity of those symptoms, to evaluate the impact of vitamin C supplementation. Seven investigators of large prophylaxis trials recorded data on symptoms which participants attributed to the medication they were using. Trials involving altogether 2,490 participants who had used more than 1 g daily of vitamin C during prophylaxis compared with 2,066 who took a placebo recorded these data. Altogether 5.8% of the vitamin C recipients reported symptoms which they attributed to the medication compared with 6.0% of those who were taking placebo (data not shown). No serious symptoms were reported. Few of the recent trials estimated the dietary intakes of vitamin C. Vitamin C is an essential nutrient and all participants had regular intakes but some lower levels than others.
None of the therapeutic trials carried out so far has examined the effect of vitamin C on children, even though the prophylaxis trials have found substantially greater effect on duration in children. In view of the greater incidence of respiratory infections in children such therapeutic trials are warranted, especially where there is known to be sub-optimal dietary intake of vitamin C.
Douglas RM, Hemilä H, Chalker E, Treacy B. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews 2007, Issue 3. Art. No.: CD000980. DOI: 10.1002/14651858.CD000980.pub3.
Note: One large, well-conducted trial reported a statistically significant but quite small reduction in common cold incidence. This trial was conducted during winter in Toronto, Canada, and participants were selected on the basis of having had problems with colds during previous winters. The relative risk observed in that trial was 0.91, the risk difference was 0.07, and thereby the number needed to treat (NNT) estimated from the study was 14. A cold Canadian winter might be a part explanation for the benefit in this trial - if it is true that cold as well as physical stress makes a prophylactic benefit for vitamin C more likely.
This trial is also interesting because it looked at the possible interaction between supplementation and dietary vitamin C levels where taking vitamin C may be most effective for those people with low amounts in their normal diet. The investigators found 48% reduction by vitamin C in ‘total days indoors’ among participants who drank less than 3 oz of fruit juices, whereas vitamin C reduced total days indoors by half the amount (22%) for those people who drank more juices. Similar modifying effect by fruit juices was found in a later therapeutic trial by the same investigators.