Can Garlic and others that must be avoided from the diet be used as medicines? (It is said that garlic milk is good to treat cough and cold).

There is absolutely no need for using garlic to treat cough and cold. There are several alternative medicines available.
As a medicine to treat cough and cold, there is no better alternative than Tulasi (Thirutuzhai). Even plucking this will yield relief. Besides this, many other medicines can be used. There is no need to use garlic as a medicine to treat cough and cold since this is classified as a food item that must be avoided.