The first-dose phenomenon is a sudden and severe fall in blood pressure that can occur when changing from a lying to a standing position the first time that an alpha blocker drug is used or when resuming the drug after many months off. This postural hypotension usually happens shortly after the first dose is absorbed into the blood and can result in syncope (fainting). Syncope occurs in approximately 1% of patients given an initial dose of 2 mg prazosin or greater. This adverse effect is self-limiting and in most cases does not recur after the initial period of therapy or during subsequent dose titration.