In most healthy men, some of the drug will remain in the body for more than two days after a single dose of tadalafil. Metabolism (clearing of the drug from the body) of tadalafil can be slowed by liver disease, kidney disease, and concurrent use of certain medications (such as erythromycin, ketoconazole, and protease inhibitors). Slowed breakdown allows tadalafil to stay in the body longer and potentially increase the risk for side effects. Therefore, doctors have to lower the dose and frequency of tadalafil in the following examples:
That means that as an adult, you need to activate the opposing parasympathetic system through sexually exciting visuals, thoughts and touches to get an erection going. But this nerve transmission is disrupted if you're stressed, anxious or distracted. The latter because you simply don't develop enough total stimulation of your genitals to get an erection, and the former because stress and anxiety all increase adrenaline – a key transmitter in the inhibiting sympathetic nervous system. They quite literally sabotage your erection.
An analysis of 14 studies involving more than 90,000 patients with ED confirmed the relation between ED and an increased risk of cardiovascular events and mortality.  Compared with patients without ED, those with ED had a 44% increased risk of cardiovascular events, a 25% increased risk of all-cause mortality, a 62% increased risk of MI, and a 39% increased risk of cerebrovascular events. Treatment of ED, either through lifestyle interventions or by pharmacologic means, may improve prognosis and reduce risk.
Erections are neurovascular events, meaning that nerves and blood vessels (arteries and veins) are involved in the process of an erection and all must work properly to develop a hard erection that lasts long enough. Erection begins with sexual stimulation. Sexual stimulation can be tactile (for example, by a partner touching the penis or by masturbation) or mental (for example, by having sexual fantasies, viewing porn). Sexual stimulation or sexual arousal causes the nerves going to the penis to release a chemical, nitric oxide. Nitric oxide increases the production of another chemical, cyclic GMP (cGMP), in the muscle of the corpora cavernosa. The cGMP causes the muscles of the corpora cavernosa to relax, and this allows more blood to flow into the penis. The incoming blood fills the corpora cavernosa, making the penis expand.
So what do you do? If the guilt is too much, you can tell your boyfriend. It might be the only way to clear your conscience — and hopefully, if you explain the reasons why you did such a boneheaded thing, he'll understand. It will be hard and it may create drama for months to come, but my guess is that you can recover, if he's not a horribly jealous guy.
The circulatory system plays a central role in obtaining and sustaining erections. Augmentation of blood flow to the corporal bodies depends on the intravascular pressure in the penile artery. Vascular lesions—typically atherosclerotic, but occasionally fibrotic—and systemic hypotension will limit flow to the corpora. In certain patients, blood flow at rest may be sufficient to obtain an erection but not sufficient to maintain it during intercourse, when the pelvic musculature places greater demands on a compromised blood supply.
Another study compared the response of surgically and medically castrated rabbits to vardenafil with that of control rabbits.  Castrated rabbits did not respond to vardenafil, whereas noncastrated rabbits did respond appropriately. This result suggests that a minimum amount of testosterone is necessary for PDE5 inhibitors to produce an erection.
The role of the endothelium in ED has been noted for a number of years and the overlapping of ED and other conditions, especially coronary heart disease, CVD, affecting endothelial function/dysfunction, is clearly present. The endothelial cell is now known to affect vascular tone and impact the process of atherosclerosis, and impacting ED, CVD and peripheral vascular disease.16
"Data shows the longer you ride, the higher your chance of developing ED in terms of distance per week," Gittens says. If you cycle for exercise or socialization, you don't have to necessarily give it up, just make some modifications. Gittens suggests riding for shorter distances, giving yourself a rest every once in a while, finding a comfortable seat, and getting a bike that’s sized appropriately.
Both ED and low testosterone (hypogonadism) increase with age. The incidence of the latter is 40% in men aged 45 years and older.  Testosterone is known to be important in mood, cognition, vitality, bone health, and muscle and fat composition. It also plays a key role in sexual dysfunction (eg, low libido, poor erection quality, ejaculatory or orgasmic dysfunction, reduced spontaneous erections, or reduced sexual activity). 
I'm a college guy who has only had sex a handful of times, and I've noticed a bit of a reccurring issue. During any foreplay and all that good stuff, I have a nice big erection, but as soon as I'm about to stick it in, the erection disappears like a frightened turtle. Then once the 30 seconds of embarrassing made-up explanations concludes, the erection is back.
Can apple cider vinegar treat erectile dysfunction? Apple cider vinegar is thought to have many health benefits, but can it help treat erectile dysfunction (ED)? ED can result from cardiovascular problems, diabetes, and other factors. Apple cider vinegar may help improve symptoms of conditions related to ED. Find out how it may help, and how to use it safely. A Home-Based Walking Program Improves Erectile Dysfunction in Men With an Acute Myocardial Infarction. The American Journal of Cardiology, 115(5), 5741-575. Retrieved from http://www.ajconline.org/article/S0002-9149(14)02270-X/abstract