I will reiterate what Tooth mentioned about the Sudafed family of drugs (as well as many other decongestants) and high PO2. Many of these drugs act like a stimulant, and the theory is that this can cause a diver to tox more easily than when not taking these medications.
Recently, when making a
big dive, we were returning to exit the cave when I experienced a block in my right ear. The way that this cave was laid out required us to descend on the return trip before we could ascend for the exit. So, I was faced with potentially having to blow out an ear drum in order to reach the cave exit and end the dive.
Luckily, I reversed my path a bit, ascended, and I was able to equalize eventually. However, I definitely was thinking about what I might have potentially had to do in order to end the dive, and this was not a pleasant thought!
If you can skip a dive and do it another day, you are better off. Ear problems can persist for a long time (I have had them last a year after the infection was over) once you have onset. It's not worth it to force a dive if you are not well.
Also regarding medications, once you have had them prescribed, it is a good idea to take them on a day when you are not going to be on a boat or out diving. Stay at home, chill out, and see if they have an adverse affect on you. If they do, you can report this to the doctor, and then the doctor can potentially prescribe something else. Better to find out on a rainy day at home than on an expensive trip!