This is largely my personal opinion, with some somewhat related experiences thrown in, but may catalyse some useful thoughts on the subject.
You need to personally look at specifics based on drugs and country.
I'd imagine that declaring what you were carrying , if in doubt, would in most cases avoid problems. I may be wrong.
Tylenol is unlikely to be a problem almost anywhere as it is very widely available and,importantly, about the only abuse it is capable of is to damage your in unpleasant ways. It happens to be one of the more dangerous drugs around while being marketed as one of the safest.
Related only - interesting factoids - which will be denied by some but which are well documented and well known by the medical establishment.
Tylenol has about the lowest ratio of harmful to maximum therapeutic dose ratio of any OTC drug. About 2:1 for most people. Approaching 1:1 for some. Under 1:1 for an unfortunate very few.
Taken at not more than its maximum allowed dose rate and duration* it has killed some people. This is rare but happens. (Being female, low weight, alcoholic or drinking heavily and with liver problems does not help). (* There is an official register of the minimum known lethal dose level for all drugs (name escapes me) - tylenol aka acetaminophen aka paracetamol has managed fatality at therapeutic dose levels).
My informal understanding is that Viagra might get you in trouble with some administrations in some cases of not declared. [As what seems a good example re a country I've never visited and am a totally non-reliable information source for - what legitimate use would a man have for viagra when entering Saudi Arabia if he was travelling without his wife?
Pseudoephedrine is a useful decongestant. To my surprise, my rather dead sense of smell improved immensely on the few occasions I have taken it. Pseudoephedrine per se is essentially harmless. Carry it undeclared or perhaps even with a prescription and you may be in trouble as it is the well known and much sought after precursor to the Class B drug amphetamine. I imagine most people know that, but odds are there are equally frowned on but essentially harmless substances that may get you in trouble. [I imagine that carrying more than 20 litres of 'Linctus G' may cause problems :-) ].
I carry a wide range of medicines when travelling. Seldom used but always wise. I tend to includes flattened packets or cutouts from packet sides of OTC medicines. For prescription medicines I try to take the container with dispensing label on + doctor's details + a prescription list provided at time of dispensing - this is not the formal prescription. I have never been asked to or needed to show any of this 'documentation' to any border official.
I may carry prescription medicine which would get one in severe trouble with authorities if carried illicitly and in greater quantities than I carry it in. I carry documentation as above. When I first travelled into China and Australia with this medication I declared it on each occasion. The Australians understood but did not care. I had trouble conveying what I was trying to say in China and nobody expressed any interest at all. I've declared it coming into NZ recently as a careful reading of the customs declaration makes it clear that I should. They also did not care. But if I do not declare this at some future stage and find myself detained and answering questions, or worse, as a result, I'd not be wholly surprised.
You'd be advised to read the forms carefully for any country you are entering or transiting and make country by country decisions accordingly. No amount of "they don't care" advice on a site like this will be of any use if "they" decide to take their own rules at face value.
Failure to comply with something 'they' do not really care about gives them a "lever" to disadvantage you with if they ever wish to for any reason. I have visited China 15+ times. On one only occasion I was given the full N person shakedown - taken to a room and EVERY item I carried in all my bags queried. Being completely 'clean' is a comfort in such situations. I found it an enjoyable joke. Being able to do so is a good idea.