ED occurs in up to 70% of men with MS, and MS is one of the most prevalent neurological disorders that affect the younger adult population worldwide (33-35). The mean time for SD and ED to develop is about 9 years and is rarely a presenting symptom of MS (36). Men with MS and ED may continue to have nocturnal erections, and psychogenic erections; however, this does not mean they have psychogenic ED but could be an indicator that MS involves the spinal cord (37).
Many reasonable nonsurgical erectile dysfunction (impotence) treatment options exist, including external vacuum devices, medications (oral and topical), hormonal therapy, penile injection therapy, and intraurethral pellet therapy. Sex counseling to further improve one's sexual health and sex life is another option and is discussed in Living With Erectile Dysfunction.
Inside the cell, NOS catalyzes the oxidation of L-arginine to NO and L-citrulline. Endogenous blockers of this pathway have been identified. The gaseous NO that is produced acts as a neurotransmitter or paracrine messenger. Its biologic half-life is only 5 seconds. NO may act within the cell or diffuse and interact with nearby target cells. In the corpora cavernosa, NO activates guanylate cyclase, which in turn increases cyclic guanosine monophosphate (cGMP). Relaxation of vascular smooth muscles by cGMP leads to vasodilation and increased blood flow.
The dorsal artery provides for engorgement of the glans during erection, whereas the bulbourethral artery supplies the bulb and the corpus spongiosum. The cavernous artery effects tumescence of the corpus cavernosum and thus is principally responsible for erection. The cavernous artery gives off many helicine arteries, which supply the trabecular erectile tissue and the sinusoids. These helicine arteries are contracted and tortuous in the flaccid state and become dilated and straight during erection. 
It is important for clinicians prescribing these drugs to make the patient aware of the action of the drugs especially the fact that they do not result in an immediate erection, and that they do not cause an erection without sexual stimulation. There is frequently a great expectation when men begin using these drugs and it is wise to temper their enthusiasm and explain they do not work immediately, and may not work every time, but also let the patient know that if these drugs do not work, there are other options.
In their extensive review, Bassil and coworkers summarise the benefits and risks, with benefits such as improvement of sexual function, bone density, muscle strength, cognition and overall improvement in quality of life. Among the risks that have been suggested include erythrocytosis, liver toxicity, worsening of sleep apnoea and cardiac function, possibly increasing symptoms of benign prostatic hyperplasia (BPH). They also note that although a possibility of stimulation of prostate cancer has been hypothesised, no scientific or clinical evidence exists to this possible risk.38
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
Specially designed vacuum devices to produce erections have been used successfully for many years. Vacuum devices are safe, relatively inexpensive, and reliable. Vacuum devices do not require surgery. Vacuum devices are available over the counter or by prescription. It is important to make sure that the vacuum device have a mechanism to prevent too high of a vacuum (negative pressure).
According to a review of all randomized controlled trials evaluating sildenafil by the American Urological Association (AUA) Consensus Panel on Erectile Dysfunction, 36% to 76% of patients receiving the drug were "able to achieve intercourse" during treatment. For tadalafil, four randomized controlled trials revealed that 11% to 47% of patients were "able to achieve intercourse." Similar efficacy has been observed with vardenafil, although studies are fewer.19 A meta-analysis published in 2013 clearly demonstrated increased efficacy over placebo for all PDE5 inhibitors.24 Head-to-head comparison suggested that tadalafil outperforms sildenafil on validated measures of erectile dysfunction, including the international index of erectile function and sexual encounter profile-2 and -3.
It appears that testosterone has NOS-independent pathways as well. In one study, castrated rats were implanted with testosterone pellets and then divided into a group that received an NOS inhibitor (L-nitro-L-arginine methyl ester [L-NAME]) and a control group that received no enzyme.  The castrated rats that were given testosterone pellets and L-NAME still had partial erections, a result suggesting the presence of a pathway independent of NOS activity.
All studies demonstrate a strong association with age, even when data are adjusted for the confounding effects of other risk factors. The independent association with aging suggests that vascular changes in the arteries and sinusoids of the corpora cavernosa, similar to those found elsewhere in the body, are contributing factors. Other risk factors associated with aging include depression, sleep apnea, and low HDL levels.
You have to desensitize yourself to stimulation. One way to do this is to masturbate about an hour beforehand, achieving another orgasm shortly after that will be more difficult. If you continue to have trouble, talk to your partner about changes to the timing of your sexual routine, you may need to give them oral or manual stimulation first or after you ejaculate, etc.