Lamar Odom -- Not the Only NBA Drug User

Lamar Odom is not the only basketball player who was high on drugs during the season -- not by a long shot, TMZ has learned.

Multiple NBA-connected sources tell TMZ ... the big problem is that the league is only allowed to test players 4 times during a season. Because testing is randomized, the last test can be administered as early as January, and the season lasts for months thereafter. As one recently-retired NBA player put it, after the 4th test, "It's like Christmas Day. We can take whatever we want."

One NBA-connected source says pot is "ubiquitous" in the sport. The source adds, however, that most athletes who indulge do so in moderation so it doesn't have a perceptible impact on their performance. An NBA rep tells us ... weed is not only prevalent, it's hardly hidden -- he says the hotel rooms where the players stay, and even the halls, have the telltale smell.

The players we've spoken with all agree ... pot is prevalent. But some of the players say Lean -- Sprite and Codeine cough syrup -- has become a recreational drug of choice during the season. One player said it was even used during the recent playoffs.

One current and famous player tells TMZ, he estimates 30% of his fellow NBAers use hard drugs -- including Molly, Ecstasy, and Lean -- at some point during the season. He says he's never been aware of anyone doing cocaine during the season.

Another current and famous player tells us he estimates only 10% of NBA players use hard drugs, and no one uses cocaine during the season -- it's too risky. He also says Lean is a drug of choice.

An NBA source agrees -- he estimates 10-15% of the players use hard drugs during the season.

One NBA insider believes the drug use problem has gotten better over the last 5 years because of social media. He says players have been burned because pictures of them doing things they shouldn't be show up on Instagram and Facebook, and it's inhibited them.