NyQuil is a combination medicine. The cough, cold and flu formula contains Tylenol (fever/pain reliever), DXM (stops cough) and an antihistamine (dries out your runny nose/postnasal drip).
NyQuil can be a great all-in-one product when it's what you need. It's useful to know what it is composed of and why. All of the medicines in it treat symptoms, not the underlying cause, which will be fought off by your immune system. If you have only one or two symptoms, you can always buy each drug separately - doing so allows you to more precisely control dosage and timing as well.
FYI, two other common symptoms not covered above are sinus congestion - which can be treated with Sudafed - and chest congestion (e.g. a phlegmy cough) - which can be treated with Mucinex.
Antihistamines do not directly dry up your runny nose/post nasal drip. Rather, they reduce the histamine response, which is helpful when something is caused by allergies. It's usually not that helpful with an actual cold (but allergic rhinitis is often mistaken for a cold).
Sudafed is what the parent was referring to with Pseudophedrine; a decongestant. This works by shrinking blood vessels, causing swollen passageways to open back up. It doesn't actually do anything to reduce the runny nose or post nasal drip, but by opening the passages more can help them drain more efficiently, preventing that feeling of congestion (hence the name).
Guaifenesin (Mucinex being the name brand) is an expectorant; it causes you to generate more mucus, and reduces the viscosity, allowing you to cough/sneeze/etc your mucus/phlegm based congestion out more easily.
Otherwise I totally agree; it is worth understanding what each of these do so you can pick and choose what you need. Nyquil includes DXM (dextromethorphan) to reduce cough severity, but the acetaminophen is not helpful unless you have a fever or headache, and the Doxylamine Succinate and Phenylephrine (both antihistamines; the Doxyl is added to Nyquil because it's also a sleep aid) aren't particularly helpful unless your cold symptoms are actually allergy related.
Better to buy DXM separately usually; fewer side effects, cheaper, and you can pair it with what else will help you (if you need something to help you sleep, you can add Doxyl or Diphenhydramine if you want; YMMV as to how effective they are)
Don't take Guaifenesin before bed (the increased mucus/phlegm production will make it harder to sleep), but it's good during daytime.
A common side effect of Sudafed is trouble sleeping; if you don't have this side effect it can be helpful in reducing congestion while you try and sleep, but if you do have this side effect, obviously, don't take it before bed.
Do any of these compounds actually do what they say? I have always found all cold medicines to be entirely worthless. Supposedly a study found guaifenesin to be no better than a placebo:
So the treatment of symptoms, in general, leads to some really inconsistent results, as it's really hard to measure them. Some studies have definitely found guaifenesin to not be helpful (others have), but the way all of them measure it is...questionable. Ultimately, what a user wants is a subjective experience of "I feel better", but what is being tested for is stuff like "what concentration of inhailed capsaicin leads to them coughing".
So generally my take is "hey, this is what it's been found effective for, and it's generally regarded as safe to take. Is it going to help here, for you, in this situation? Who knows! Give it a whirl if you got the money and want to try".
Guaifenesin is mostly useless and only approved so that they can stick it next to your dextromethorphan as an emetic to stop you from abusing it. I was vomiting from COVID and I was incredibly pissed off when I found out it was actually just because they poisoned my cough syrup on purpose.
Prior to the regulation of pseudoephedrine, NyQuil also contained pseudoephedrine and since the nasal congestion from a a virus can often be the most disruptive factor for sleep (at least, in my personal experience, that's the case) it's considered by some to the be most important component.
NyQuil with pseudoephedrine included is now marketed as "NyQuil D" and is available behind many pharmacy counters with the same restrictions as other pseudoephedrine products. However, it could also rightly be called "NyQuil Classic" (to borrow branding from Coca-Cola).
While true, the cocaine is processed out of the leaves themselves and sold to pharmaceutical companies. The remaining leaf product is used in the flavoring.
I’m not sure if it is still legal, but importing “coca tea” - that is, tea bags filled with shredded coca leaves - was legal at some point and a few enterprising folks imported a few kilos of it and processed it into the drug.
It's still legal to buy decocanized coca leaves but I think the odds of getting anything worthwhile out of it would be slim and probably more expensive than a plane ticket to Peru.
NyQuil also contains alcohol and pseudo (in some formulations).
My pet theory is that NyQuil's biggest effect is simply to make you mildly "faded" so all your symptoms are more tolerable. DXM, the antihistamine, the pseudo, and the alcohol are all drugs that would definitely do that if taken in larger doses. While NyQuil doesn't have those larger doses, the combination of all of them may amplify the otherwise weaker effects into a general buzz/haze that helps you go about your day/night.
Sort of! There isn't consistent evidence that -any- antitussive is more effective than placebo. A majority of studies show a statistically significant result for DXM, but not all, and they aren't fully replicable which definitely calls it into question. But that's true of every purported cough suppressant, and there's understandable reasons for that; coughing is a voluntary response to irritation, so there's definitely a lot going to determine whether you cough or not on beyond a purely autonomous system response. Plus there's not really much clarity in how to measure improvement (reduction in frequency may not actually be a reduction in irritation; reduction in severity is hard to measure. Etc).
So, really, for a given incident, try it, see if it helps. If it does, great, if it doesn't, stop taking it.
NyQuil can be a great all-in-one product when it's what you need. It's useful to know what it is composed of and why. All of the medicines in it treat symptoms, not the underlying cause, which will be fought off by your immune system. If you have only one or two symptoms, you can always buy each drug separately - doing so allows you to more precisely control dosage and timing as well.
FYI, two other common symptoms not covered above are sinus congestion - which can be treated with Sudafed - and chest congestion (e.g. a phlegmy cough) - which can be treated with Mucinex.