When you go shoe shopping you wouldn’t pick up a size 13 shoe when you wear size 8. So why do so many men buy condoms that don’t fit?

If you wear a condom that is too small it can be very uncomfortable; too large and the condom can slip right off.  Wearing a condom that doesn’t fit quite right doesn’t just make sex uncomfortable but also increases the risk of unintended pregnancy or contracting a sexually transmitted disease.

Wearing poorly- fitting condoms more than doubles the risk of breakage, slippage, erection loss and difficulty in reach an orgasm by either the user or his partner. Ill-fitting condoms are also five times more likely to cause irritation of the penis.

“STDs do occur as a direct consequence to putting on condoms late because they don’t like it or taking off condoms too early,” said Richard Crosby, professor and chair in the Department of Health Behavior at the University of Kentucky, in an interview with TheCheckup.

So what condom size is best for you?

In the United States, condoms are limited in size but Crosby expects the market will soon adapt to the varying needs of men.

“Newer companies are increasingly recognizing that to accommodate men’s size there needs to be dozens of sizes available,” Crosby said. “A company recently hit the European market with 95 different sizes. It comes with a kit for men to use to measure the size and circumference of an erect penis to find the fit that is best for them.”

Until a product like that hits the American market, men will have to fit into one of three sizes – slim fit, large and extra large.

Crosby said the perfect fitting condom should fit snuggly on the penis without extra space except for tip, which should have room for the ejaculate.

“Unfortunately too many men make purchasing decisions based on ego. A man may have an extra large ego but a medium or small penis,” he said. “The reverse problem is not uncommon when a man has a large or extra large penis and tries to fit a small or regular size condom. The condom will work the same but the result is that the man will not feel comfortable in that condom.”

In 2010, Crosby conducted a study, which found about 45 percent of men used ill-fitting condoms. The study was on a small scale with only 436 men ages 18 to 67 self-reporting.

In that study, Crosby noted that the issue of fit wasn’t just affecting the men.

“These are issues women understand about from their male partners,” he said. “Couples should be actively involved in finding the right size condom that will benefit both people.”