If patients discover that they have missed a dose by an hour or so, it is usually best for them to go ahead and take the missed dosage right away. They can then get back on schedule as quickly as possible (by taking the next dosage at the same time they would have had they not missed the time) so that there is only one aberration rather than several.
However, if patients miss the dose by around 5 hours or more one might consider having them go ahead and take the medicine at that time, and then take a number of subsequent doses at equal intervals that are less than 12 hours until the patients are back on schedule. If the patients are without complaints, one might consider having the patients eliminate the missed dose completely, while staying on schedule by taking the next dose at the same time they would have had they not missed the dose. Generally, the former alternative is better in patients who are less clinically stable.