Marijuana. Mug shot removal services. Lottery tickets. Gambling. Online porn. Try to make one of these purchases with your credit card and you will likely be denied.

Because of the complex laws governing these industries, credit card issuers would often rather play it safe than risk getting in trouble.

American Express, for example, says it considers all of these products and services either "illegal or high-risk."

"It's a matter of these banks being a little leery to get into some of these areas where there is ambiguity about law or areas that the public may see as unsavory or unseemly," said Matt Schulz, a senior industry analyst at CreditCards.com.

Here are five transactions that you'll have a hard time putting on the plastic.

Mug shot removal

Websites claiming to remove online mug shots have come under fire by consumer advocates, lawyers and state legislators in recent years because they require people to pay hundreds, or even, thousands of dollars to remove their photos from the sites.

And in many cases, the company offering the photo removal service was believed to be the same one that published the photo in the first place.

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Because of the controversy surrounding these services, some credit card companies have decided to steer clear of the industry. Discover (DFS) and American Express (AXP) both said they do not permit transactions for these services.

Visa (V) said that while some of these sites may be illegal, some could also be legal, so the bank issuing the card would need to be careful to allow only legal transactions. MasterCard (MA) declined to comment on the specific items that it declines, but said its rules "require our customers to conduct lawful activity where they are licensed to use our brands."

Pot

While marijuana is now legal (in some form) in more than 20 states, it is still considered an illegal substance under federal law.

That leaves credit card companies in a tricky place. For now, most of the major issuers are refusing to process any transactions involving marijuana -- even if it's considered legal by the state.

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Discover said it won't allow the purchase of marijuana for either medical or recreational use because the purchases aren't legal under federal law. "We continue to monitor this topic closely," a Discover spokeswoman said.

American Express also refuses to process pot transactions. And Visa said it only allows these purchases where pot is legal, but the ultimate decision to approve or deny a transaction must be made by the customer's bank.

Online porn

Because of the controversial nature of online pornography, at least one card issuer, American Express, doesn't want anything to do with it. It prohibits cardholders from purchasing adult entertainment online.

Discover, however, allows customers to purchase legal adult porn. Visa doesn't block porn purchases either, but says individual banks can decide to deny these transactions if they deem them to be too risky.

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Gambling

"[Gambling] is similar to marijuana in that it's one of those categories where the legislation, laws and restrictions around it are so confusing that some banks are just staying out of it entirely," said Schulz.

American Express and Discover both refuse to process purchases of gambling chips in the United States -- online and at casinos.

Visa said that since there are some legal forms of gambling, and because legality also depends on the state or jurisdiction, it's up to the customer's bank to approve or deny individual transactions.

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Lottery tickets

Since it's technically a form of gambling, lotteries can also be tricky for card issuers.

Discover says it prohibits purchases of lottery tickets anywhere in the United States, as does American Express.

Visa, however, will process lottery ticket transactions as long as the lottery program is legal.

For all of these industries, Visa says its policy is to prohibit any transactions that are clearly illegal. But if there's a "gray area" and the legality is not so cut and dry, it's up to individual banks to use their own judgment when deciding what to allow and prohibit.