Prazosin disposition in young and elderly subjects.

1 The disposition of prazosin following oral and intravenous administration has been studied in seven young and seven elderly men, all of whom were in good health and living independently in the community. 2 The elimination half-life (min) of prazosin was 123 +/- 19.4 (s.d.) in the young and 194 +/- 36 in the elderly (P less than 0.01). 3 Clearance (ml min-1 kg-1) was 3.94 +/- 0.73 in the young and 3.53 +/- 1 in the elderly (P greater than 0.1). Volume of distribution at steady state (1 kg-1) was 0.63 +/- 0.14 in the young and 0.89 +/- 0.26 in the elderly (P less than 0.05). 4 The absolute bioavailability of orally administered prazosin was 0.68 +/- 0.17 in the young and 0.48 +/- 0.16 in the elderly (P less than 0.05). 5 The significant increase in prazosin half-life in the elderly results from an increased volume of distribution, not from decreased clearance. 6 Gastrointestinal absorption of prazosin decreases in old age.