Personally I have over the years taken several meds that, according to the number of doses per day, should ideally be taken every 8 hours. With most of them, though, it has been fine for me to take them with 4-6 hour gaps, and a longer break overnight while I (try to) sleep.
If in doubt, though, you can check with a pharmacist. They are usually very knowledgeable about the meds they prescribe, and if they're not sure have reference books they can check in. Some meds it is important to keep the right gap, others, so long as you don't exceed the daily dose, the gaps between doses are less important.
HMS (diagnosed 13/9/12 confirmed at UCHL 27/11/12), ME, FMS, autoimmune hypothyroidism
"The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it." (Terry Pratchett)