This is the 805th most frequent Finnish word.
viikossa
"In a week" or "per week."
Here, 'viikossa' is used to describe the frequency of an action within the time frame of a week.
Käyn kuntosalilla kolme kertaa viikossa.
I go to the gym three times a week.
In this sentence, 'viikossa' indicates the context of calculating an average number related to weeks.
Keskiarvo on viisi lukuviikossa.
The average is five reading weeks.
'Viikossa' is detailing the schedule or interval at which the medicine is consumed, which is once per week.