Antihistamines block histamine, a substance that's released in response to allergies and causes a runny nose and sneezing. Probably aren't useful during a cold unless you also have allergies. Antitussives tell your brain to stop coughing. Useful for a dry cough. Don't take an antitussive if you're coughing up mucus.
Expectorants help thin mucus so it can be coughed up more easily.
Doctors disagree about whether they work. Drinking lots of fluid is one of the best ways to thin mucus. Nasal decongestant sprays shrink nasal passages. We recommend that you do not use these. One of most useful remedies for a cold.