Several years ago (and this isn't too recent, don't recall the details) a night carrier landing accdient was determined to have been the result of impairment by the pilot. The incapacitation, or impairment, was the result of the use of an over-the-counter nasal medication; a spray. I believe it was afrin.
The FAA does not approve medications. The FAA has a short list of medications which have been determined to have no general side effects which present a psyiological hazard for flying. However, the FAA does not approve these medications for use when flying. Further, the FAA holds that of equal or greater significance is the fact that the medication must be taken in the first place.
The medication may not be disqualifying in that it may have no significant side effects, but the condition itself may certainly be disqualifying under 14 CFR 61.53 (see below).
I can tell you I've had emergency hospitalization following a sinus implosion that occured while flying as a passenger, and it was the most intense pain I've ever experienced. This has happened twice. The first time I had to visit the ER twice, and the second time years later, I was deaf for a month. In the first case, I had no idea I was sick; I had no symptoms. This is quite similiar to presenting symptoms and hiding them with medication. The result may be the same. The stuffy nose may be gone, but the sinus is still a ticking bomb. This applies to many conditions.
I suggest the following links:
http://www.leftseat.com/medcat.htm
http://www.aviationmedicine.com/linkaero.htm
§ 61.53 Prohibition on operations during medical deficiency.
(a) Operations that require a medical certificate. Except as provided for in paragraph (b) of this section, a person who holds a current medical certificate issued under part 67 of this chapter shall not act as pilot in command, or in any other capacity as a required pilot flight crewmember, while that person:
(1) Knows or has reason to know of any medical condition that would make the person unable to meet the requirements for the medical certificate necessary for the pilot operation; or
(2) Is taking medication or receiving other treatment for a medical condition that results in the person being unable to meet the requirements for the medical certificate necessary for the pilot operation.
(b) Operations that do not require a medical certificate. For operations provided for in § 61.23(b) of this part, a person shall not act as pilot in command, or in any other capacity as a required pilot flight crewmember, while that person knows or has reason to know of any medical condition that would make the person unable to operate the aircraft in a safe manner.