For freely available medication the full price has to be paid by the patient. The pharmacist informs you about the dosage of the medication and possible side effects.
Medication that needs a prescription is free of charge if it is fully covered by your insurer. If your insurer does not cover the complete costs of the medication you will be charged a part of the price. However, this is not too much compared to the total price.
Keep in mind that prescriptions have validity dates. Prescriptions from emergency services are only valid for one day after their issuance, prescriptions for antibiotics are valid for three days and all others are valid for one week.
To save costs the Czech government has published a list of drugs that are provided free on insurance. Only the cheapest drug of one substance group is fully paid for by the insurer unless your doctor proves that there is no alternative to a more expensive product.