Men's health expert Dr. Harry Fisch, author of The Male Biological Clock and professor of urology at Columbia University in New York, is co-author of the new book Size Matters: The Hard Facts About Male Sexuality That Every Woman Should Know. Here, he discusses how to prolong an erection, what coffee does to a man's sex drive, and the most dangerous sexual position.

Q: What is normal when it comes to penis size?

A: It's a little bit more than five inches. The bottom line is that nearly everyone is between four and six inches. I always joke that it's the width of the book. I always talk about norms to highlight the abnormal, which is if you have a penis size of less than three inches erect. Most penises are roughly the same size. If you have a small guy or a big guy – you really can't tell.

Q: Can you explain the ways in which size actually does matter?

A: Here's where the title came from. I wrote a book called the Male Biological Clock before this, and I'm a men's health expert. I'm a professor of urology at Columbia. It turns out, there are definitely some things where size matters. The size of the belly, for example – the bigger the belly, the lower the testosterone levels. There's a direct correlation. If a man has increased the size of his belly over the years, his testosterone level has gone done. If he gets into shape and loses that belly fat, his testosterone level will rise. The size of a man's belly should be half his height.

The second thing has to do with a man's testicles. Size absolutely matters for fertility. When I see a man who has very small testicles, chances are he'll have a very low sperm count. The testicles make sperm. They should also be symmetrical – they should be the same size. And if he has a lopsided scrotum where one side is hanging much lower than the other, that shouldn't be. It can be associated with varicose veins in the scrotum, which not only decreases sperm count but also decreases testosterone levels.

Q: In the book, you talk about premature ejaculation and you say that a lot of women are surprised to learn how long a man is supposed to last. How long?

A: The number one sexual dysfunction is premature ejaculation. Less than two minutes is premature ejaculation, severe premature ejaculation is less than one minute – and that's after penetration. If you ejaculate before penetration, that's really severe. The average is between five and 10 minutes. Between two and five minutes, you're in a grey zone.

Q: If you're in that grey zone, how can your partner delay ejaculation?

A: There are a lot of things we talk about in the book that are non-medical: stop/start techniques, squeezing the base of the penis, thinking about other things, using condoms. But those aren't very romantic. What works best are medications. Anti-depressants at very low doses have the side effect of prolonging an erection. There's also a Lanacane jelly, but the only problem with it is, if you don't use a condom, the woman becomes numb so what good is that?

Q: What about during oral sex, when you want to get things moving?

A: (Laughs) I've never had anyone ask me to how to speed things up. I don't have an answer for that one. Maybe more foreplay beforehand, which increases the seminal fluid. Ejaculation is really a reflex action. When there's more fluid in the prostate area, then there's a reflex ejaculation.

Q: There's a pervasive perception that men want to have sex all of the time. If your dude tells you he's too tired for sex, should you be worried?

A: Yes. People are supposed to have sex. If a man is tired, it could be sign of low testosterone, fatigue or anxiety. A lot of people have better sex in the morning than at night because there's less anxiety. For example, if a guy gets up at 5:30am and is getting his butt kicked all day, I can see where he might say that he's too tired – but that's indicating a stress factor. It's a medical issue if he's too tired to have sex.

Q: What about the connection between the things he consumes and his libido?

A: Excessive coffee drinking will limit your ability to have sex. It's the adrenalin. People also take decongestants, like Sudafed, and they have a very hard time having sex. When people have prolonged erections, we treat them with Sudafed to bring down the erections. It allows the blood to empty from the penis more quickly. For women, Sudafed and coffee may mean they have less of an ability to have an orgasm.

Q: What impact does alcohol have on penis function?

A: Oh, big problem. One drink for me, I'm buzzed. But I go to the Jets games and there are guys drinking gallons of beer. I'm looking at them and I can't believe what I'm watching. Everybody's different. A very little amount of alcohol relaxes you; too much, it's a depressant which will limit your ability to have sex.

Q: What about food?

A: A lot of food will limit sexual function because you have all of the blood in your belly. You need the blood in your penis. Here's what happens: You eat a lot of food, you get tired, you drink some coffee and you've already had a couple of drinks. If you have a cold and also take some Sudafed, you're finished.

Q: So you should keep him sober and slightly underfed?

A: That's exactly right.

Q: Ok, I have to throw in a bonus question. Do men demonstrate any general preference when it comes to sexual positions?

A: More people use the missionary position – especially when you're trying to have a child. When it comes to recreational sex, I don't know of any study that's looked into which position people prefer. Some men might prefer having sex from behind because there's less interaction, you're not making eye contact. If you're in the missionary position, you're looking at each other. The position that causes the most trauma to the penis is woman-on-top. The penis can slip out and fracture, and then you need a surgical procedure to correct it.

What would you ask Dr. Fisch if you had the chance? Submit your questions below in the comment section.