Pharmacies are open during usual shop hours and you will be required to pay out of pocket for medicines.
Pharmacies (Apotek) are present all around the country and are open during usual shop opening hours (10:00-18:00 on weekdays and 10:00-14:00 on Saturdays, this can vary locally). In every major city you will be able to find a 24h pharmacy. The 24h health information number is the 1177.
Most medicines, except benign one such as aspirins, need a prescription from your doctor. You will need to pay your medicines in Sweden. If you are a Swedish resident, the amount you pay will be topped at a ceiling of SEK 1,800 during each twelve month period (does not apply to European health card holders).