Continue to take sildenafil even if you feel well. Mean sildenafil plasma concentrations measured after the administration of a single oral dose of mg to healthy male volunteers is depicted below: Figure 5: Mean Sildenafil Plasma Concentrations in Healthy Male Volunteers. The combination can cause a dangerous fall in your blood pressure. Talk with your healthcare provider about which might be best for you. 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. It is not recommended for use in Australia. Topicals tend to have a lower risk of side effects compared to oral medications, Dr.Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil . Viagra is absorbed quickly into your body. It can start working within 30 minutes, but it may take a couple hours for some people. However, one study on Viagra found that about 70% of men achieved erections in 30 minutes, and more than 80% of men achieved erections within 45 minutes. For most men, the effects of Viagra will start within . Rarely reported side effects include: an erection that will not go away (priapism). If you have an erection that lasts more than 4 hours, get medical help right away. If it is not treated right away, priapism can permanently damage your penis. sudden vision loss in one or both eyes.