I have used Sildenafil before (Sildenafil is the medication name, but Viagra is the corporate brand name).
Keep in mind that its not an ED medicine, it is a blood medication. It opens up your arteries so there is more bloodflow (not just to your dick but your whole body). The bloodflow will also go to your dick and give you an erection.
I used higher doses before, 50mg and 100mg, and it worked well in keeping an erection. I think 25mg is not a strong enough dose, at least for me personally, it could be different for other men.
I had weird side effects when I used Sildenafil such as chest pain, headache, and eye pain. Other than that, no other problems for me.