Dr. Ken, A few different friends of mine who are in their late 20s have mentioned using Viagra or Cialis for better sex. I'm thinking of giving it a shot. Any reason not to?

-- Joey P. in Las Vegas

Joey, Men 18 to 45 are the fastest-growing group receiving erectile dysfunction (ED) medicine prescriptions. Apparently your friends are adding to that emerging demographic. But first an important digression: Erections often seem like mysterious things. Let me explain how they happen and how the drugs give you a rise.

(Should you use it? See the complete answer after the jump.)

Erections occur when there's more blood flowing into the penis than out.

The active ingredient in ED medicines elevates the level of a compound called Cyclic GMP that prevents blood from flowing out -- acting as a kind of penile dam that allows the penis to become engorged.

Who needs these drugs? Those with conditions that can cause poor blood flow. A diabetic, for example. When a person has impaired blood flow, such a drug can aid erections. In an otherwise healthy young buck, however, it will similarly alter blood flow.

Should you use it? There's no evidence of immediate or long-term harm from Cialis, Levitra or Viagra when used recreationally. However, this doesn't mean there isn't potential for harm.

As with any medicine, there are side effects to be aware of: ED medicines can lower your blood pressure and cause dizziness, light-headedness and fainting. They can affect your vision and even make you see everything with a blue tint for a few hours.

On the scary side, there is an association with sudden death (though this is rare and likely due to other factors such as a heart condition). Last but not least is priapism -- an erection lasting four or more hours. (And the problem is? Pain. This marathon erection hurts, and you'll be praying for it to end. Oh, cruel irony.)

One other finding of note: Recreational use of ED medicines has been associated with a higher risk of acquiring STDs and HIV. I use the phrase "associated with" not "caused by." The medicine did not cause the infections, however recreational use tends to be done in conjunction with other high-risk behaviors such as no or improper condom use.

My advice: Unless your doctor is telling you otherwise, leave the pharmaceutical companies out of your sex activities. But don't leave out your jimmy hat.

Dr. Ken Spaeth is a physician and Research Fellow at the Harvard School of Public Health. He is also co-author of the Bioterrorism Sourcebook.
You can e-mail him your questions at askdrken@aol.com