An uncontrolled, unstoppable erection may sound like a fantasy to most men, especially those in middle age and beyond that are struggling with erectile dysfunction. However, any erection over a couple of hours is termed priapism and, besides being very painful, can result in permanent damage to the penis. (Always go to the emergency room immediately if you cannot stop an erection!)
What causes priapism? Surprisingly, priapism is not completely understood. However, there are many rather suprising triggers of priapism that many men m
1. Alcohol. Heavy drinking was one of the most common associations with priapism.  Alcohol results in vasodilation - although if you have too much it can have the opposite vasoconstricting effect - and the right dosage apparently can overstimulate. Probably more importantly, alcohol affects the alpha adrenergic system - more about that below. 
2. Trazodone. This antidepressant is estimated to cause 80% of drug-induced priapisms and can result in some rather severe cases.  One particularly gruesome case resulted in amputation of the penis.  If you have been prescribed this drug, beware that "in approximately 33% of the cases reported, surgical intervention was required and, in a portion of these cases, permanent impairment of erectile function or impotence resulted." 
What causes such severe cases? Trazodone is "thought to result from unopposed ?-adrenergic blockade; pharmacologic treatment consists of infusion of ?-adrenergic agonist drugs directly into the penis." The significance is that noradrenaline actually blocks an erection from occurring. In fact, surgeons have injected noradrenaline directly into the penis in order to bring down erections. 
One can think of nitric oxide as the stimulator of erections and, in a way, noradrenaline as the dampener. When you remove the brakes, the truck just keeps rolling down the hill with nothing to stop it. Thus, alpha adrenergic antagonists (alpha-adrenoceptor antagonists), which decrease noradrenaline levels, are very often the root cause of priapism.
3. Erectile Supplements and Herbs. Most herbal and supplement impotence preparations do not have the ingredients listed. And, even if they do, can in some men induce priapism. One severe case involved a preparation called "All Nite Long" and the researchers reported that "a cavernoglanular shunt was then attempted with a large biopsy needle (Winter shunt) and subsequently with a scalpel (Ebbehoj shunt), without success."  Needles and scalpels - it hurts just to think about it!
Yohimbe (Yohimbine) is also common such preparations and can lead to priapism.  Why is that? One compounnd in yohimbine is an alpha adrenergic antagonist. One poster on our forum also mentioned another "formula" that resulted in the same. Did it also have yohimbine in it? Who knows?
Of course, the tragedy is that many men are taking these concoctions in the hopes of helping their sex lives only to find that they have potentially damaged themselves permanently.
4. Alpha Blockers. Did you know that there are a whole class of drugs that are, by definition, alpha adrenoceptor antagonists? And, yes, these can cause priapism. Again, middle-aged and senior men are particularly vulnerable, because these are commonly prescribed for BPH (enlarged prostate). One nasty case of this is mentioned in the literature, where a man's priapism persisted even after shunt surgery on his penis.  Prazosin has also been linked to the same issue. 
5. Cocaine and Prescription Narcotics. Cocaine is notorious for priapism and, again, the reason is related: it depletes the body of noradrenaline. As noradrenaline (norepinephrine) levels fall, eventually the body loses its ability to shut off an erection in many cases.
This can happen with prescription narcotics (vicodin, percoset, oxycodone, etc.) as well. One strange story out of the U.K. in late 2011 tells of a man who, due to a medical condition, supposedly experiened painful long-lasting erections and then robbed his own father at knife point to get money to buy pain killers.  We ask the question, "Could he not have been first addicted to pain killers which then led to his priapism?"
6. Pomegranate Juice and Viagra. There have been several case reports of priapism resulting from this combination. Pomegranate juice is not only a potent stimulator of nitric oxide but can alter liver function. Those taking any PDE5 inhibitor (Cialis, Levitra, Stendra, etc.) should also use caution. 
2) Alcohol Clin Exp Res, 2009 Feb, 33(2):255-63, "A pilot trial of the alpha-1 adrenergic antagonist, prazosin, for alcohol dependence"
3) http://www.medscape.com/viewarticle/549253_6, "Counseling Patients About Sexual Issues: Drug-Induced Priapism"
4) J Clin Psychiatry, 2010, 12(2), "Penile Amputation After Trazodone-Induced Priapism: A Case Report"
6) International Journal of Impotence Research, 2005, 17:469â€“470, "Priapism caused by 'All Nite Long'"
7) Eur Urol, 1986, 12(3):169-70, "Intracavernous injection of noradrenaline to interrupt erections during surgical interventions"
8) JOURNAL OF MEDICAL TOXICOLOGY, 5(4):223-225, "Refractory priapism associated with ingestion of yohimbe extract"
9) International Journal of Impotence Research, 2005, 17:293â€“294, "Unresolved priapism secondary to tamsulosin"
10) Br Med J, 1979 October 27, 2(6197):1039, "Prazosin and priapism"