A feared Mexican drug lord who made an unusual addition to Forbes' list of top billionaires has been dropped from the latest ranking because his assets are hard to calculate, the magazine said.

The 2013 list, which came out Monday with another Mexican, telecoms tycoon Carlos Slim, at the top, has "kicked out" Joaquin "El Chapo" Guzman, the business magazine said.

The Sinaloa drug cartel leader has been in hiding since escaping from a Mexican maximum security prison in 2001. Reports late last month that he was killed during a shootout between traffickers in a jungle region on the Guatemala-Mexico border were never confirmed.

However, the uncertainty surrounding the allegedly brutal narco baron was enough to have him tossed from a list where he rubbed shoulders with the likes of Slim and Microsoft founder Bill Gates.

"Although he is believed to likely be alive and living in Mexico, Guzman's whereabouts are unknown," Forbes wealth editor Luisa Kroll said.

"Also sketchy: how he's spending the illicit money he earns. Forbes has been unable to reach him to verify figures and believes an increasing chunk of money is going to protect him and his family.

"As the leader of the Sinaloa cartel, he is one of the most powerful people in the world, but no longer someone we are confident enough to call a billionaire."