They shuffle around, hunched forward, with a full on stiffy straining against their trousers, drooling and grinning like a madman as they desperately seek some form of sexual release, trees, lamposts, cars - anything will do.
This lasts for around 8 hours, plus a slight blue tinge to their vision and headaches the next morning, elevated blood pressure is also commonly noted along with thick hairs sprouting from their ears and nostrils.
Conversely, they might find it easier to gain and maintain a stiffer erection with less effort, although consideration must also be given to the hightened mental state - 'I've taken a magic sex pill, phwoorrr, I'm such a super stud' - increasing their hopes, dreams and desires of having a great shag - or two.
They will also find it easier to regain an erection after the first orgasm with a shorter recovery period. Orgasms are often delayed and Viagra is often used to help train premature ejaculation suffers to cope with their condition better.
Viagra should always be supplied with lube, imho, as you probably will need it sooner or later in the session.