Definition and discussion
Dosing interval (symbol: τ, tau) is the time interval between the administration of individual doses of a drug. For example, if 1 tablet is administered at 8 AM and 1 tablet is administered at 8 PM, the dosing interval is 12 hours. The typical dosing interval is several hours or one day. However, there are also drugs that need to be administered infrequently. These include, for example, bisphosphonates for the prevention and treatment of osteoporosis. Depending on the drug, they can be administered daily, weekly, monthly, every three months or even once a year. A long dosing interval can be an advantage for treatment adherence. However, administration may be forgotten if infrequent. The dosing interval depends on the dosage form, pharmacokinetics, and elimination of the drug. One expression for elimination is the half-life. The shorter the half-life, the shorter the dosing interval accordingly. For example, the painkiller ibuprofen has a half-life in the range of one to three hours. Accordingly, it must be taken between three and four times a day for pain. The dosing interval can be extended through sustained release, which is 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, accumulation and adverse effects may occur. If, on the other hand, it is too long, the steady state cannot be achieved. With a single dose, there is no dosing interval.
Illustration
Once daily
In once-daily administration, a drug is administered every 24 hours and always at the same time of day, for example, at 8 am.
Twice daily
Twice daily usually means that a drug is taken 12 hours apart, for example, at 8 am in the morning and 8 pm in the evening. However, there are also medications that are given in the morning and at noon, for example. With this instruction, it is always necessary to specify.
Three times a day
In a thrice-daily administration, the drug is in principle administered every 8 hours, for example, in the morning at 8 am, at 4 pm, and at night at midnight. Because this is impractical, the drug is often given morning, noon, and night. But then the dosing interval is of different lengths.
Four times a day
Four times a day basically means that a drug is given every 6 hours, for example, at 8 am, 2 pm, 8 pm, and 2 am. In practice, for example, pain medications are often taken in the morning, at noon, in the evening, and before bedtime (see Figure). This is permissible for paracetamol, for example, for which the dosing interval should be at least 4 to 8 hours according to the specialist information. In the example shown above, the dosing interval is long at night, which could lead to a loss of efficacy. During the day, on the other hand, it is rather short in some cases.
