Ah, cold season. If you are not being bombarded with woo regarding vitamin C or zinc someone is likely telling you another one of my old favorites: if you do have a cold or allergies you need to avoid dairy products. Is there any evidence to support this?
First of all, if you have already been skeptical, or you just wanted to have hot chocolate with milk because you thought it might be comforting, you're in luck. There have been multiple studies that indicate that there is no evidence that consumption of dairy products significantly increases production of mucus. (Link)
There are some people that do have a milk protein allergy that do experience respiratory problems when they consume milk, but if don't already have problems with dairy products, there's no reason to avoid them when you have a cold.
As far as why this myth is still around, for those of you who are interested, this is a great in example of a logical fallacy know as The Appeal to Ancient Wisdom. In other words, since the avoidance of dairy products for respiratory difficulties has been mentioned by the Jewish physician Moses Maimonides and in Traditional Chinese medicine, people think it must be true because it's old (Link). If this were really the case, then we would still need to engage in the practice of trepanning to cure headaches, and we would also believe that the earth was flat as well. Old doesn't mean better, period.
Why do some people think they get increased mucus/phlegm when they consume dairy products? One of my theories is that one of the most common symptoms that people start with in the cycle of a common cold is a sore throat, so some people attempt to soothe their sore throat with cold things like ice cream, pudding, etc. Increased nasal secretions will usually appear on day 3-5 of the cold, which of course happens after the dairy products have been consumed. By day 6-7 of the cold, people are starting to feel better, and they might put it down to ceasing their consumption of dairy, but this is also normal as far as a decrease in cold symptoms. This brings us to another type of logical fallacy know as post hoc ergo propter hoc, or "because I did x, then y happens." Remember, correlation does not equal causation.
Take home message--if you are someone who has to avoid dairy products for other reasons or you are one of the few people with a milk protein allergy, you probably need to keep doing so. No reason to change from your usual dairy consumption if you have a cold.