How many medications do you take each day

Dosing intervalPharmaWikiThe dosage interval is the time between the administration of the individual doses of a drug.

synonymous: dew

Definition and discussion

The time between the administration of the individual doses of a drug is referred to as the dosing interval (symbol: τ, tau). For example, if 1 tablet is administered in the morning at 8 a.m. and in the evening at 8 p.m., the dosing interval is 12 hours.

The typical dosing interval is several hours or a day. However, there are also drugs that rarely need to be administered. These include, for example, the bisphosphonates for the prevention and treatment of osteoporosis. Depending on the preparation, they can be administered daily, weekly, monthly, every three months or even once a year.

A long dosing interval can be beneficial to compliance. However, the administration can also be forgotten if it is seldom done.

The dosage interval depends on the dosage form, the pharmacokinetics and the elimination of the active ingredient. An expression for the elimination is the half-life. The shorter the half-life, the shorter the dosing interval. For example, the pain reliever ibuprofen has a half-life in the range of one to three hours. Accordingly, it has to be taken three to four times a day if you are in pain. The dosing interval can be extended by means of a retardation, i.e. the slow and continuous release from the dosage form.

An adequate dosing interval is a prerequisite for reaching steady state. If it is too short, it can lead to accumulation and undesirable effects. On the other hand, if it is too long, the steady state cannot be reached. There is no dosage interval for a single dose.


Ingestion of drugs and dosage interval, click to enlarge. Illustration © PharmaWiki

Once a day

In the case of once-a-day administration, a drug is administered every 24 hours and always at the same time of day, e.g. at 8 a.m.

Twice daily

Twice a day usually means that a drug is taken every 12 hours, e.g. in the morning at 8 a.m. and in the evening at 8 p.m. But there are also medicines that are administered in the morning and at noon, for example. Clarification is always required with this instruction.

Three times a day

When administered three times a day, the drug is in principle administered every 8 hours, for example in the morning at 8 a.m., at 4 p.m. and at night at midnight. Often times, since this is impractical, the medicine is given in the morning, noon, and evening. Then the dosing interval is of different length.

Four times a day

Four times a day basically means that a drug is administered every 6 hours, for example at 8 a.m., 2 p.m., 8 p.m. and 2 a.m. In practice, painkillers, for example, are often taken in the morning, at noon, in the evening and before going to bed (see illustration). This is permissible, for example, for paracetamol, for which the dosage interval should be at least 4 to 8 hours according to the technical information. In the example shown above, the dosing interval is long at night, which could lead to a loss of effectiveness. During the day, however, it is sometimes rather short.

see also literature
  • Medicinal product information (CH)
  • Specialist literature
  • Pharmacotherapy textbooks

Conflicts of Interest: None / Independent. The author has no relationships with the manufacturers and is not involved in the sale of the products mentioned.

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This article was last changed on January 13th, 2020.
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