If you want to take high doses it can be taken until you develop loose stools, then lower the doses. Most experts think vit c is more effective when taken with rose hips or bioflavanoids. I've read that vit C is most effective when taken at the start of the cold, not when the congestion has developed.

When I have a really bad cold with lots of congestion I take echinacea, small amounts of Vit C, L-Lysine. I've found that crushed bay leaves - inhale the aroma - or put some in hot water and inhale is good for breaking up congestion as is eucylyptis oil, inhaled. Any cough medicine with hoarhound works well for me.

Hope this helps.

Don't just take vitamins c, there is also over dosage in vit c. instead why not do some home remedies like steaming, or taking juices.in my case provn to help alleviate colds? is vicks and humidifier plus drink lots of fluids specially citrus juices.