Ahh, winter.

The leaves have fallen, the breeze is chilly… and cold season has arrived. When that sticky cough and sniffly nose hits, you probably cozy up with some hot soup, a box of tissues, and your favorite brand of cold medicine.

There are syrups, pills, and vaporizers to treat about every symptom your cold can bring, and they all involve some cool chemistry. But before you down that bitter syrup, it might also help to know that not every cold medicine is as effective as you’d think. Colds are some of the most common infectious diseases we can get, but since they’re caused by more than 200 different viruses, it’s been basically impossible for us to create a vaccine against them.

So the best we can do for now is fight the symptoms. Most cold symptoms are probably caused by your body’s efforts to fight off the virus, not the virus itself. When you catch a cold, inflammatory proteins like cytokines, get released and tell your immune cells to get to work fixing the problem.

Mainly, they do this by dilating your blood vessels so that infection-fighting white blood cells can move around more easily. Unfortunately, in your nose, those dilated blood vessels also lead to tissue swelling and congestion. Cytokines also interact with pain receptors in your esophagus, which can cause a sore throat.

And in the nervous system, they can stimulate various nerves to trigger a runny nose, sneezing and coughing. None of those things are especially fun -- okay, really, they kind of stink. So we’ve come up with all kinds of drugs to knock them down as much as possible.

When your nose is stuffed up, you’ll probably reach for a decongestant to clear up your sinuses. Pseudoephedrine is a popular one, found in medications like Sudafed Decongestant and Aleve D. It’s a carbon ring attached to a complex chain of other atoms, including nitrogen and oxygen.

It reduces congestion by binding to receptors in your nose, which makes the muscles around your blood vessels tense up and reduces the extra blood flow caused by cytokines. Unfortunately, since pseudoephedrine is a stimulant, it can also can cause insomnia, nervousness, dizziness, and even affect your heart rate. It’s even been used Breaking Bad-style to make methamphetamines illegally, which is why it’s kept behind the pharmacy counter.

Phenylephrine is a similar medicine, found in brands like Sudafed PE, but since it can’t be used to make meth, it’s easier to buy. Unfortunately, several studies have also found that it doesn’t seem to be any more effective than a placebo, which means it probably doesn’t do much. So, if that’s your favorite decongestant… sorry.

Some colds can also cause chest congestion and coughing. To clear out some of that mucus, many medications contain an expectorant, which helps you spit up those lovely green gobs of phlegm. The most common one is called guaifenesin, which is a carbon ring attached to chains of oxygen, hydrogen, and carbon atoms. It’s found in medicines like Mucinex.

We’re not entirely sure why it works, but scientists think it suppresses the production of mucins, one of the main proteins in the mucus lining your lungs. That makes it easier for specialized cells to move mucus up and out of your body. Still, that’s just based on research done in a lab.

In patients, scientists have found that guaifenesin does seems to help thin mucus, but it might not be that effective at clearing it from your lungs. Some studies haven’t found much of an improvement over a placebo in people with respiratory infections. But at least we know it helps somehow.

Now, maybe the most irritating symptom of a cold -- for you and everyone around you -- is that nasty, lasting cough. To help calm it, many medications, including Robitussin, contain a cough suppressant, like dextromethorphan. Dextromethorphan and molecules like it are made up of four carbon rings, along with some oxygen and nitrogen atoms.

They’re derived from morphine, a highly-addictive painkiller, but at the doses used in cold medicines, they don’t have the same potential for abuse. We’re not 100% sure how these molecules work, but they probably work on the brain instead of the respiratory system. When you take dextromethorphan, it crosses the blood-brain barrier -- the tightly-woven net of cells in your brain’s blood vessels that protect it from bacteria and infection.

And once it’s there, it affects a bunch of different receptors. Researchers think that it helps reduce coughing by binding to receptors for neurotransmitters called NMDA, sigma-1, and serotonin, since other drugs that bind to them also seem to suppress coughing. Mainly, this tricks your brain into thinking there’s no tickle in your throat and that you don’t really need to cough.

Then again, some recent studies have also found that dextromethorphan may not actually be that effective, at least in kids, although many studies don’t seem to be that reliable. Outside of cold medicine, though, it does have some uses. Because of its effects on the nervous system, it’s also being investigated for use in a bunch of different conditions, like treating anxiety and agitation in Alzheimer's disease -- which is really cool.

Still, effective or not, all of these medications only treat the symptoms of the cold. None of them actually do anything to help kill the virus or boost your immune system -- except maybe by helping you sleep better. Ultimately, the only thing that can cure a cold is time!

But these medicines will hopefully make things easier while you wait it out. Thanks for watching this episode of SciShow! Besides over-the-counter medicines, you’ve probably heard how vitamin C or zinc can help stop a cold -- but that’s not true.

And if you’d like to learn more, you can watch our episode all about it to find out why.