Patients often ask the question – why do I have to take this medicine once daily and that medicine four times daily? They might even wonder what four times daily really means – does it refer to waking hours or does it refer to the entire 24-hour clock?

First and foremost, let’s consider half-life.

The half-life of a drug is, to put it simply, the amount of time it takes for the body to eliminate half of the amount of that drug from the body. Hypothetically speaking, then, a drug with a half-life of 4 hours loses 75 percent of its initial administered dose after 8 hours.

Drugs with longer half-lives require less dosing; it takes longer for the body to remove the existing dose from the body. In contrast, drugs with shorter half-lives require more frequent dosing because the body eliminates the drug at a faster rate.

In other words, drugs that need to be administered four times daily have shorter half-lives. This has clinical consequences, too. For example, patients are more likely to take a drug once daily rather than four times daily. In other words, they show greater adherence.

That deals with the first half of our question. Next, we need to assess the times that ‘four times daily’, and its sister doses, refer to.

When to take medicines

It often depends on the medicine itself.

Medicines usually say ‘four times daily’, rather than ‘every 6 hours’. Typically, unless otherwise instructed, this refers to four times administration during your normal waking hours. Strictly adhering to the ‘hours’ format increases the risk of missed doses.

The purpose of regular dosing, at regular and defined intervals, is to keep a consistent drug concentration inside the body. This is important in cases of, for example, infection – where lower drug concentrations may precipitate an infectious relapse.

Four-times daily dosing, then, could be administered as follows: once in the morning and once in the late evening, with the other two doses spaced evenly between these periods. Patients accord their own times in conjunction with their typical daily schedule.

Once daily dosing can be deceptive, though. Some patients may assume that they can administer the dose at a time of their choosing. For drug concentration reasons, though, patients should take the medicine at the same time each day.

What matters is that the doses are taken, but you should always follow the instructions in the patient information leaflet, as well as from your healthcare provider and/or pharmacist.



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