As I often state, an erection is actually a very complex phyical process that involves multiple bodily systems. This is why optimal general health is so critical for optimum bedroom performance (and libido). Furthermore, few men realize that many of the things, lifestyle and otherwise, are very significant erection killers. This can be in the short term or even lead to longer term erectile dysfunction as we will discuss below.
Most of the list below is common sense. However, a lot of times when you are in the battle, it is hard some of the important details that could be causing you issues. Plus, many things that can hurt your general and sexual health are culturally acceptable and so it simply never crosses our minds. So, if you are experiencing erectile difficulties, scan through the list of 15 Erection Killers below to see if any may apply and then discuss with your physician:
CAUTION: Do not alter any medications without talking to your physician first.
1. Low Testosterone. Okay, this is no big shock. However, I put it first because many men do not realize that it can lead to Venous Leakage, an intimidating condition where a man has great difficulty maintaining an erection.
2. Loss of Morning Erections. Okay, this is no big shock. For more information, see my link on The Basics of Morning Erections, an intimidating condition where a man has great difficulty maintaining an erection. It doesn't matter how much nitric oxide you have if your penis cannot "lock and hold" the blood flow into the corpus cavernosum. If you are not getting morning erections, you need to make sure you get regular erections somehow, some way as this is critical for the health of the penis.
3. High Blood Pressure. High blood pressure is significantly associated with erectile dysfunction and the reasons are many. First of all, it is actually a root cause of endothelial damage, because higher pressures can lead to damaged arterial walls. However, hypertension is also a symptom of the Metabolic Sndrome, a suite of symptoms that includes insulin resistance and elevated triglycerides as well. For more information, see my link on Erectile Dysfunction and High Blood Pressure.
4. All Cardiovascular Risk Factors. Almost anything that is hard on the heart and cardiovascular system is going to lead to erectile dysfunction. The reason is simple: erectile dysfunction is strongly related to endothelial dysfunction where the delicate lining of your arteries becomes damaged and does not output nitric oxide like it should. This means that things like elevated homocysteine, which have only been found to indirectly lead to erection problems, will likely soon be found to be linked to impotence. We will give numerous examples below. to erectile dysfunction. And, yes, this is the primary reason that lifestyle is so critical: exercise, weight management and diet all powerfully affect all of these. Don't believe me? Just look at the title of this study: "Endothelial dysfunction links erectile dysfunction to heart disease" 
5. Cholesterol and LDL. Cholesterol has been linked in many studies to erectile dysfunction as I document in my link on Cholesterol and Erectile Dysfunction. The reason is not actually cholesterol directly but rather the fact that elevated cholesterol is linked to elevated LDL, which, when oxidized, damages arteries. Low HDL leves have been linked as well.
6. Bike Seats. What could be healthier than bike riding, eh? It turns out that many standard bike seats can damage the pudendal nerve that is connected to the penis. If you have done a lot of bike riding recently and have erectile dysfunction, this is definitely something to consider. The solution is as simple as changing out your bike seat. For more information, see my link on Nerve Damage and Erectile Dysfunction.
7. Bisphenol-A and Phthalates. A lot of guys think that environmental toxins are grossly overblown and could not possibly kill their erections. All I can say is ignore all the research at your own peril. Do you seriously think that you can avoid the onslaught of endocrine disruptors and xenoestrogens that litter our modern environment? This simply is not Kansas any more and many, many studies have shown how these can damage the reproductive and sexual function of animals. Now there is evidence breaking out that they can do the same in men. See my links on Bisphenol-A and Phthalates for examples of erectile dysfunction and effemization studies for example.
8. Prolactin. Elevated prolactin is surprisingly common. If it gets high enough, it can lower testosterone and cause a number of male issues. One study noted that "about 80% of men with prolactin greater than 50 ng./ml. complain of diminished libido and erectile dysfunction because excess prolactin can suppress secretion of gonadotropin-releasing hormone."  See my link on Prolactin and Prolactinomas for additional information.
9. Estradiol (E2). Low testosterone can negatively impact erections, so why not too much estradiol, the "bad" estrogen? This definitely has been shown to be the case in clinical practice, where many HRT clinics find that they not only need to raise T sufficiently but keep estradiol in the 20-30 range. Many men go on injections and cannot figure out why they are struggling even though their testosterone has doubled or tripled. The reason, more often than not, is that estradiol has also been driven up proportionately and is sabotaging the results. If this is your situation, find a knowledgeable physician that will monitor both your T and E2.
10. Overweight. Okay, this is no big shock: those extra pounds can hammer erections in even the most mighty Weekend Warrior. Being overweight does every nasty thing that you can think of: it lowers testosterone, increases estrogen, raises inflammation, boost insulin levels - the list goes on and on. To see just what extra body weight does to you, check out these links on How Being Overweight Affects Erections and What Those Extra Pounds Do To You.
11. Smoking. Interestingly enough, smoking seems to raise testosterone just a little. But that's the last of the good news: cigarette smoke is VERY hard on the endothelium. In fact, nicotine is a vasoconstrictor, i.e. narrows your arteries allowing less blood to flow to the penis. However, it is the smoke that really causes the damage. One research study explained that "Nicotine and carbon monoxide are much less damaging than is whole smoke. However the free radical components of cigarette smoke have been shown to cause damage in model systems." 
12. Medications. Many, many medications will kill erections. Some lower testosterone. Some raise prolactin. Some have a sedating effect by raising seratonin levels. Some alter dopamine function. The bottom line is that if you are on a medication and having erectile difficulties, this is one of the first places that you should look. I have some starter information in my links on Testosterone and Drugs and Erectile Dysfunction and Drugs.
13. Depression. Being depressed is miserable. In fact, it's actually painful and anyone that has experienced it knows just what I am talking about. Unfortunately, to add insult to injury, it is also very hard on erections. You can read about it here in my links on Testosterone and Depression and Erectile Dysfunctiona and Depression.
14. Recreational Drugs. A lot of men do not realize that even something as relatively benign as marijuana use can affect their sex life. Don't believe me? Check on my link on Testosterone and Marijuana. Of course, painkillers of all shapes and sizes as well as cocaine and herone can really kill erections. See these links on Cocaine and Heroin and The Problem with Painkillers.
1) International Journal of Clinical Practice, Feb 2005, 59(2):225 229, "Endothelial dysfunction links erectile dysfunction to heart disease"
2) The Journal of Urology, Jun 2003, 169(6):2262 2264, "Laboratory Evaluations of Erectile Dysfunction: An Evidence Based Approach"
3) International Journal of Experimental Pathology, Aug 2000, 81(4):219 230, "Cigarette smoking, endothelial injury and cardiovascular disease"