The head of the NHS drug watchdog said it would not be forced into approving medicines it had doubts over yesterday as the body came under fresh pressure over the use of a controversial Alzheimer's treatment.

Andrew Dillon, chief executive of the National Institute for Clinical Excellence (Nice), said it was right to rule that the drugs cost too much and do not work well for most Alzheimer's patients and warned that having to fight a legal battle would tie up time and resources that should be spent ensuring the NHS gets treatments that are value for money.

But in an interview with the Guardian, he conceded that Nice's constant and public battle with drug companies and patient groups could potentially undermine its standing. "There is a danger that continued criticism can become corrosive," said Mr Dillon. "But how do we deal with that? We can't say 'well we're going to have to make a few less controversial decisions for the next six months so that we take the heat off us'."

Nice has been the target of sustained pressure from drug companies and patient groups over a number of decisions. Most recently, it was crticised for ruling that drugs to treat Alzheimer's can be prescribed on the NHS only in the moderate stage of the disease, not when symptoms begin.

Nice has been accused of betraying patients, cruelty, lack of ethics and "victimisation of the most vulnerable in society". Mr Dillon said it was his job to remain calm and rational, and not for Nice to "fight like a tabloid editor with those with whom we might have an issue".

He added: "I've certainly been disappointed by some of the statements that are made by those with whom we work, but I've accepted that that is a product sometimes of the high emotion generated by our activities. It is a difficult judgment really because on the one hand I might feel pretty offended by it but I don't think I can go out shouting. I think I've got to go out and say look - it's not right for these reasons, this is how we work, this is the job we have to do."

Eisai and Pfizer, the two drug companies who make and market Alzheimer's drug Aricept are applying for a judicial review of the decision and the Alzheimer's Society announced yesterday it would join the action. In an official statement, Mr Dillon said: "We consider [Esai's] claim without foundation." Controversy over decisions to ban some drugs from the NHS and limit others has raged since Nice was launched in 1999, but Mr Dillon says it does not get to him personally.

"We quite understand that if you spent $800m (£412m) in developing a product you're going to feel very strongly and equally if you are a national organisation promoting the interests of a particular patient group you're going to be passionate, and so I'm neither surprised by nor offended by the language."

Pharmaceutical companies lobby ministers about Nice's decisions. Recently the US deputy health secretary, Alex Azar, explained his thinking on the errors of rationing mechanisms like Nice to the health secretary, Patricia Hewitt, and his conviction that a free market for the largely US-based major drug companies was a better idea.