The pattern of responses was similar for the other principal question, the ability to achieve an erection sufficient for intercourse. All medications must be declared upon arrival at the border. It does, however, give law-abiding Americans some options if they can’t get or can’t afford medications in the U. In a study of healthy male volunteers, sildenafil mg did not affect the steady state pharmacokinetics of the HIV protease inhibitors, saquinavir and ritonavir, both of which are CYP3A4 substrates. Therefore, inhibitors of these isoenzymes may reduce sildenafil clearance and inducers of these isoenzymes may increase sildenafil clearance. In addition, you must state, in writing, that the medication is for your personal use. Mutagenesis Sildenafil was negative in in vitro bacterial and Chinese hamster ovary cell assays to detect mutagenicity, and in vitro human lymphocytes and in vivo mouse micronucleus assays to detect clastogenicity.Sildenafil is a generic drug, which is an exact copy of the active drug in a brand-name medication. The generic is considered to be as safe and effective as the original drug. And generics tend to. Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision. In most males,* Viagra works within 1 hour after it’s taken. It’s possible that Viagra can begin working within 30 minutes of being taken. But other times, it may take up to 4 hours to start.