It will probably not surpise most of you that most drugs are very hard on your penis considering how hard they are on your body. Well, consider your suspicions confirmed. Most guys using recreational drugs do not wake up in the morning saying, "Today, I'm going to trade a half hour of pleasure for my sex life." But that is exactly what most recreational drug users aren doing without even realizing it. It may take a week or two or may take a year or two, but almost always one of the first things to go out the window are the ol' erections.
Let's start with the opiates, such as heroin, methadone and so on. All of these are very hard on male erectile strength, sabotaging it in many different ways. Some of these drugs should be called Erection Killer, Anti-Viagra, Castratanol or something similar. One study looked at heroin addicts in treatment with methadone or buprenorphine and found that almost half reported erectile dysfunction, 24% saying mild to moderate and 18% severe.  Of course, that assumes that you survive: cocaine also is a fantastic clot builder and artery spasmer and heart attacks are not
NOTE: Don't think you are immune if you're abusing prescription drugs. The DEA estimates that in America there are "nearly 7 million Americans are abusing prescription drugs more than the number who are abusing cocaine, heroin, hallucinogens, Ecstasy, and inhalants, combined". Furthermore, the problem is bound to get worse as one recent study in the Journal of Adolescent Health found that one in five adolescents lends or borrows prescription drugs. Prescription drugs are just as powerful and damaging as those on the street.
But how does cocaine lead to erectile dysfunction? One journal points out that it basically greatly accelerates the aging of the arteries, leading to accelerated plaques and calcification.  Another study found that chronic cocaine use lead to chronically elevated CRP levels, which good Peak Testosterone readers know is one of the leading risk factors for heart disease.  Methadone and heroin destroy just about every marker of gonad function that researchers can think of. It lowered ejaculatory volume, seminal vesicular and prostatic secretions by over half!  Those Nirvana lyrics "Half the Man I Used to Be" really apply here. But the nightmare doesn't stop there: baseline testosterone levels were 43% lower in methadone users for example. Interestingly enough, methadone (a controlled substance given at clinics) produces the worst results in all markers, but heroin is not far behind.
Amphetamines may be called uppers, but they should be called downers, as in hang downers, when it comes to your penis. Amphetamines do the same as the opiates: accelerating the aging of the arterial and vascular walls with plaques quickly leading to endothelial dysfunction.  The body has certain specs. You can't run most car engines at 10,000 rpm and it's the same with your body - push it too hard and you will damage it. Get ready to limp through life - and limp is the best word here - if you believe that the high and the energy of amphetamines is worth the cost.
Next we will cover marijuana. This supposedly benign plant that everyone wants to use for medicinal purposes should be called The Great Sterilizer. Please read this link on How Marijuana Can Destroy Your Hormones, Fertility and Lead to Impotence.
1) Drug and Alcohol Dependencies, Apr 1 2008, p. 12-18
2) Med J Australia, 2002, 177(5):260-262
3) Cardiology, 88(3):191
4) NEJM, Apr 24 1975, 292:882-887
5) J of Postgraduate Med, 2006, 52(1):51-56