i thought cialis lasted 36 hours.
Sorry I misspoke, the T1/2 is the half life, it is 17.5 hours for cialis. That means it takes 17.5 hours for the blood level to drop by half. The time to steady state in the blood is 3.6 half-lives, (principle of pharmacology), so that would be 3 days. That is for the 5 mg daily dose. The duration of action of a single dose is 72 hours. That is the time that 1/2 the dose is still in your blood and still as effective for ED as the intermittent ED dose of 10-20 mg. The half-life is longer for use in pulmonary hypertension (lower levels needed for vasodilation here). Sorry for the error. The point is given it several days to reach a steady state and achieve the full effect on the daily dose.
For intermittent use for ED, the difference between Cialis and Viagra is as follows:
Viagra is much shorter acting lasting only about 4 hours (half-life of 4 hours).
for Cialis, he Onset of action is 30 min but may take up to 2 hrs. for maximum effect. Viagra is much quicker and begins to work in 30-45 min for maximum effect for ED.
That's why Viagra may work if taken shortly before sex, but for cialis, many men prefer taking it the night before and you're ready to go for 72 hours.
Everyone's individual experience is different and ymmv as we say.