After ejaculation with viagra 25mg, i return to flacid state, but wake up with semi-hard erections to urinate during nite, which also subside. worry?

Penis health : Frankly I would not worry, a semi soft erection or an erection during the night is a normal process. I would worry if the erection is firm and lasting after sex and ejaculation. This would suggest that Viagra is working too much as a vasodilator and can be very unhealthy for the health of your penis plus cardiovascular system.

Nope; however...: This is what Viagra does. No need to worry. If you don't like it consider natural alternatives to Viagra, such as L-Arginine 2000 mg and Eklonia Cava (sold as Seanol), a seaweed extract that can be very helpful for erectile dysfunction- see https://draxe.com/natural-remedies-impotence/ and http://www.eckloniacava.com/erectiledifficulties.php