mania:...As a generally healthy person I stick to this rule - so I don't take pain killers when diving (even if I have a headache evening before), I don't take seasickness medicine etc. But if you have to take medicine - check it first with the appriopriate MD...
...For me and strictly for me the fact that it may cause nausea is enough reason not to dive while taking doxycycline...
That rule is okay as a personal rule , but everyone should be aware that most pain medications will be out of our systems in 8 hours. Taking a couple of Tylenol or Motrin, or even Vicodin the night before for a headache or chronic pain won't effect or be effected by your dive. Seasickness medicine isn't even a problem. In fact, a lot of doctors will recommend meclizine during dive trips because of the motion of the boat and the potential for seasickness. No worries during the dive because the reason you're taking it is on the boat not under water. In other words, you shouldn't be afraid to take medicine just because you're diving. If you would normally take it anyway and you know it won't be bad to take while diving, then take it.
Almost every drug out there MAY cause nausea in someone. Drug companies are required to state this if even just one person became nauseated during trials.
I'm not big on taking meds myself, but some people need them for real medical reasons and shouldn't be afraid to take their meds on dive trips.