If you're looking for a little online titillation, don't use your American Express card for the experience.The company doesn't allow its cardholders to purchase online pornography, even though it's federally legal, says Chao.

It doesn't matter if you're buying marijuana for a medical purpose or for recreation; the transaction can only be made by cash.

Discover and American Express have confirmed they don't allow cardholders to buy medical or recreational marijuana."It's our policy to adhere to federal law in such matters," says Sanette Chao, a spokeswoman for American Express.

Federal law still bans the sale of marijuana for medical or recreational purposes, even though 18 states and Washington, D.

Credit card brands always brag about their cards' ability to expand your purchasing power and vie with each other for the greatest universal acceptance worldwide. It turns out there are some things credit cards won't buy.

Remember Master Card's priceless slogan: "There are some things money can't buy. So you leave home without it, to riff off of American Express' catchphrase.

While credit companies typically want you to pull out their credit card for a purchase, the payment networks -- American Express, Discover, Master Card and Visa -- won't approve so-called naughty purchases.These transactions are treated as high-risk activities or run afoul of federal law, even if state laws allow it.C., have legalized medical marijuana, and two states -- Washington and Colorado -- recently OK'd recreational marijuana purchases.Other credit card companies have also placed restrictions on the use of credit cards for marijuana purchases, while the payment processors -- such as Visa and Master Card -- have canceled merchant agreements with medical marijuana providers across the country, says Betty Aldworth, deputy director of the National Cannabis Industry Association.Visa and Master Card did not respond to requests for comment.Aldworth also says banks won't extend basic banking services to marijuana dispensaries, meaning they can't take checks, either."This can cause real-life issues for patients desperately seeking medicinal marijuana," she says.