A single iodine dose provides protection for one day and some protection on the second day after taking it. The iodine dose should be taken 1–6 hours before exposure to radioactive iodine in order to provide full protection. The later the iodine dose is taken, the greater the reduction in the protective effect. The iodine dose is of no use if the radioactive substances were inhaled more than 12 hours before taking the dose. To ensure correct timing, the iodine tablet should only be taken as instructed by the authorities.