High dose, but not low dose, zinc lozenges shorten the duration of cold symptoms. A review of all previous studies on the topic showed that the reason why some showed no benefit is because the dosage used matters. It takes at least 75 mg of zinc acetate or gluconate for it to work (glycinate and citrate forms of zinc do not work). Daily doses exceeding 75 mg per day showed a 42% reduction in duration of colds.
The specific effects of zinc include anti-rhinoviral properties, interferon induction (protects cells from viral invasion), and immune boosting.
Other general treatments for colds is reviewed and include high dose vitamin D, vitamin C, maitake mushrooms, vitamin A, and supportive care that includes a humidifyer, nettie pot, ginger tea with lemon and manuka honey, and photonic stimulation.