The mechanisms by which testosterone plays a role in erectile function are not completely understood. A study evaluating the effect of testosterone on erections in surgically castrated rabbits and control animals, in which the rabbits’ intracavernosal pressures were compared after cavernosal nerve stimulation, determined that castrated rabbits had much lower pressures after stimulation than control rabbits did.  Notably, the pressures increased when castrated rabbits received exogenous testosterone replacement.
The Massachusetts Male Aging Study (MMAS) documented an inverse correlation between ED risk and high-density lipoprotein (HDL) cholesterol levels but did not identify any effect from elevated total cholesterol levels.  Another study involving male subjects aged 45-54 years found a correlation with abnormal HDL cholesterol levels but also found a correlation with elevated total cholesterol levels. The MMAS included a preponderance of older men.
In this study, ED proceeded CVD in almost 70% of cases. Similarly, many men with ED have been found to have pre-existing CVD. A study by Vlachopoulos et al evaluated the incidence of asymptomatic CVD in 50 men with ED.22 These authors found that 19% of men with ED had asymptomatic CVD. Similarly, Mulhall and colleagues found that 20% of men presenting with ED and vascular insufficiency on penile duplex had asymptomatic CVD.23
Sildenafil has been previously suggested as a treatment option for ED in men with epilepsy (77,78). However, Matos et al. warned that PDE5i are potentially pro-convulsant and should be used with great caution in men with epilepsy (79). Animal studies in rat and mice overwhelmingly suggest PDE5i can reduce seizure threshold. In human trials, seizures were rare but reported. PDE5i exerted their proconvulsive effect by lower seizure threshold possibly by worsening sleep or obstructive sleep apnea, causing cardiovascular changes, or leading to EEG changes specifically with tadalafil use.
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). 
The vascular processes that produce an erection are controlled by the nervous system and certain prescription medications may have the side effect of interfering with necessary nerve signals. Among the possible culprits are a variety of stimulants, sedatives, diuretics, antihistamines, and drugs to treat high blood pressure, cancer, or depression. But never stop a medication unless your doctor tells you to. In addition, alcohol, tobacco, and illegal drugs, such as marijuana, may contribute to the dysfunction.
Alprostadil is injected into the side of penis with a very fine needle. It's of great value to have the first shot in the doctor's office before doing this on your own. Self-injection lessons should be given in your doctor's office by an experienced professional. The success rate for getting an erection firm enough to have sex is as high as 85% with this treatment. Many men who do not respond to oral PDE5 inhibitors can be ‘rescued' with ICI.