On 2017 Oct 11, Harri Hemila commented:

Zinc lozenges and vitamin C are promising approaches for the common cold

Papadopoulos NG, 2017 reviewed promising approaches for the treatment and prevention of viral respiratory tract infections, but did not include any meta-analyses or trial reports on zinc lozenges and vitamin C.

Three randomized trials using zinc acetate lozenges found that common cold duration was shortened by 2.7 days (95% CI: 1.8-3.3 days), Hemilä H, 2016, and that the rate of recovery from colds was increased 3.1-fold (2.1-4.7), Hemilä H, 2017. Although zinc lozenges are dissolved in the oro-pharyngeal region, they also shortened the duration of symptoms in the nasal region: nasal discharge by 34% (17-51%) and nasal congestion by 37% (15-58%), Hemilä H, 2015.

The binding of zinc to acetate is weaker than to gluconate and there have been suggestions that zinc acetate might therefore be a better salt for lozenges. Nevertheless, it is not evident that differences at the chemical level between these salts are translated into substantial differences at the clinical level. A meta-analysis found no evidence that lozenges formulated from either of these 2 salts differed in their effects on common cold duration, Hemilä H, 2017. In addition, 2 zinc gluconate trials found that the rate of recovery from colds was influenced to a similar extent as that for the 3 zinc acetate trials, Hemilä H, 2017.

In the trials that reported benefits of zinc lozenges, the total daily dose of elemental zinc was over 75 mg/day. This dose is higher than the recommended intake of 11 mg/day for men and 8 mg/day for women in the USA. However, 100-150 mg/day of zinc has been administered to certain patient groups for months with few adverse effects, and 150 mg/day of zinc is currently one standard treatment for Wilson’s disease, Hemilä H, 2017. Thus, it seems unlikely that 80-100 mg/day of zinc for about a week for treating a cold might lead to long-term adverse effects. There are therefore good reasons for classifying zinc lozenges as a promising approach to treat colds.

A Cochrane review showed with a narrow confidence interval that vitamin C does not prevent colds in the general community, Hemilä H, 2013. However, a meta-analysis of 5 randomized trials with participants under heavy short-term physical stress showed that vitamin C decreased the incidence of colds by 52% (36-65%). Thus, vitamin C seems to have prophylactic effects albeit only in special conditions.

A meta-analysis of more than 2 dozen placebo-controlled trials calculated that ≥1 g/day of vitamin C shortens the duration of colds in children by 18% (9-27%) and in adults by 8% (4-12%), Hemilä H, 2013. Furthermore, 2 trials found a linear dose-response association in the effects on common cold duration by up to 6- and 8-g/day of vitamin C, with colds being shortened by about 20% with the higher dose, Hemilä H, 2017. Still higher therapeutic dosages might lead to greater benefits but such dosages have not been investigated yet. More research also on vitamin C is evidently warranted.

On 2017 Oct 11, Harri Hemila commented:

Zinc lozenges and vitamin C are promising approaches for the common cold

Papadopoulos NG, 2017 reviewed promising approaches for the treatment and prevention of viral respiratory tract infections, but did not include any meta-analyses or trial reports on zinc lozenges and vitamin C.

Three randomized trials using zinc acetate lozenges found that common cold duration was shortened by 2.7 days (95% CI: 1.8-3.3 days), Hemilä H, 2016, and that the rate of recovery from colds was increased 3.1-fold (2.1-4.7), Hemilä H, 2017. Although zinc lozenges are dissolved in the oro-pharyngeal region, they also shortened the duration of symptoms in the nasal region: nasal discharge by 34% (17-51%) and nasal congestion by 37% (15-58%), Hemilä H, 2015.

The binding of zinc to acetate is weaker than to gluconate and there have been suggestions that zinc acetate might therefore be a better salt for lozenges. Nevertheless, it is not evident that differences at the chemical level between these salts are translated into substantial differences at the clinical level. A meta-analysis found no evidence that lozenges formulated from either of these 2 salts differed in their effects on common cold duration, Hemilä H, 2017. In addition, 2 zinc gluconate trials found that the rate of recovery from colds was influenced to a similar extent as that for the 3 zinc acetate trials, Hemilä H, 2017.

In the trials that reported benefits of zinc lozenges, the total daily dose of elemental zinc was over 75 mg/day. This dose is higher than the recommended intake of 11 mg/day for men and 8 mg/day for women in the USA. However, 100-150 mg/day of zinc has been administered to certain patient groups for months with few adverse effects, and 150 mg/day of zinc is currently one standard treatment for Wilson’s disease, Hemilä H, 2017. Thus, it seems unlikely that 80-100 mg/day of zinc for about a week for treating a cold might lead to long-term adverse effects. There are therefore good reasons for classifying zinc lozenges as a promising approach to treat colds.

A Cochrane review showed with a narrow confidence interval that vitamin C does not prevent colds in the general community, Hemilä H, 2013. However, a meta-analysis of 5 randomized trials with participants under heavy short-term physical stress showed that vitamin C decreased the incidence of colds by 52% (36-65%). Thus, vitamin C seems to have prophylactic effects albeit only in special conditions.

A meta-analysis of more than 2 dozen placebo-controlled trials calculated that ≥1 g/day of vitamin C shortens the duration of colds in children by 18% (9-27%) and in adults by 8% (4-12%), Hemilä H, 2013. Furthermore, 2 trials found a linear dose-response association in the effects on common cold duration by up to 6- and 8-g/day of vitamin C, with colds being shortened by about 20% with the higher dose, Hemilä H, 2017. Still higher therapeutic dosages might lead to greater benefits but such dosages have not been investigated yet. More research also on vitamin C is evidently warranted.