The Spaniard, who was capped 40 times and retired last year after five seasons on Teesside, also accused former manager Gareth Southgate of keeping him in the dark.
The midfielder signed from Lazio in 2003 and helped Boro lift their first-ever trophy a year later when they won the Carling Cup. But the 35-year-old is horrified at the club's slump into the Championship, and he vented his anger in a Madrid newspaper.
Accuser: Gaizka Mendieta, now retired, claims Boro had a shocking booze culture
'At Middlesbrough the beer used to flow in the dressing-room the moment matches were over,' he said.
Stoke City striker Kitson joins Middlesbrough on two-month loan Bruce lines up £5m switch to take Boro defender Wheater to SunderlandEXCLUSIVE: Steve McClaren is back: Former England manager rebuilds his career in Holland 'The following day the players were supposed to work on their recovery but instead they'd just have a coffee and then get back on the booze once more.
'They used to eat Mars bars and chocolates, and when I said something about it they'd fire back, claiming it was to give them energy.
Accused: Mendieta blames former boss Southgate
'You could say that there was a clash of attitudes between me and them.
'My first two or three years at the club were very good. We won a trophy and got to the UEFA Cup final. But everything went wrong once Steve McClaren left and Gareth Southgate replaced him as manager.
'We had played together and got on well, but suddenly he told me I was not part of his plans.
'The one thing I reproach him for is the fact that he was never honest with me. The club wanted to go in the direction of using youngsters, and get rid of a handful of players.
'I had offers to join clubs in Spain and England on loan. But I did not want to go away for six months, only to find myself in the same situation once more.'
Stoke City striker Kitson joins Middlesbrough on two-month loan Bruce lines up £5m switch to take Boro defender Wheater to SunderlandEXCLUSIVE: Steve McClaren is back: Former England manager rebuilds his career in Holland