According to doctors and pharmacists, this is the most common--and one of the most dangerous--medication errors people can make, and yet it happens every day.
To help make this clear, I've put together a list of common OTC cold and flu 
- Comtrex (325 mg acetominophen)
- Nyquil (500 mg acetominophenl)
- Dayquil (325 mg acetominophen)
- Dristan cold (325 acetominophen)
- Nurofen (200 mg ibuprofen)
- Contac (500 mg acetominophen)
- Alka Seltzer Plus (250 mg acetominophen)
As you can see, acetominophen (Tylenol) or acetominophen is in a lot of combo products. This makes it all too easy to take your regular dose of acetominophen for your headache or aches and pains, then double dose by taking a cold
Another concern: Acetominophen can be tough on the liver, so if you take a double dose and then drink alcohol (remember, Nyquil and many cough syrups contain alcohol) you're loading up your liver in a way that can be unhealthy.
So, what to do? As a shortcut, watch out for any product labeled "multi-symptom" and always read the ingredients. Space your doses of acetominophen four hours apart, whether it's in a pain reliever or a combination cold and flu medicine. Space doses of ibuprofen six hours apart. And best not to drink alcohol, or limit yourself to one drink, when taking these remedies.