The European Commission says its president Jean-Claude Juncker is taking medication for acute leg pain which made him stumble during a Nato summit - and it insists that he was not drunk.
It was "a particularly painful attack of sciatica, accompanied by cramps".
Mr Juncker, 63, was filmed by news agency AP struggling to keep his balance before a gala dinner in Brussels on Wednesday.
Looking wobbly, he was helped by the Dutch and Portuguese leaders.
When a reporter asked if Mr Juncker had been drunk the Commission spokesman replied that the question was "more than tasteless". "I don't think this is elegant, I don't think this is fair," he said.
After the attack, the spokesman said, Mr Juncker "attended the dinner normally and then the pain disappeared". "His programme is full. His programme has not changed at all."
This is not the first time he has been filmed unsteady on his feet. The former prime minister of Luxembourg has blamed sciatica in the past too.
The UK's NHS website says sciatica symptoms can include pain, numbness or tingling in the buttocks, legs and feet. It is caused by compression or irritation of the sciatic nerve.
Pictures taken the same night show Mr Juncker being brought into the dinner in a wheelchair.
According to VRT, Dutch Prime Minister Mark Rutte, one of the leaders seen supporting Mr Juncker, told reporters he was unaware of any "serious health problems" but said he was aware that Mr Juncker had "had a back problem for a while".
At the start of a speech to the Irish parliament last month Mr Juncker blamed his rather wobbly steps on sciatica.
"I have some difficulties to walk. I am not drunk. I have sciatica. I would prefer to be drunk. Hah!"
Juncker denies 'drink rumours'
In 2016, Mr Juncker dismissed speculation that he had an alcohol problem. In an interview with Brussels commentator Jean Quatremer in the French daily Liberation he said such "rumours" were spread by his opponents.
Quatremer wrote that "during our recent lunch he downed four glasses of champagne to go with just a salad".
He then quoted Mr Juncker as saying: "Do you think I'd still be in my job if I was hitting the cognac at breakfast time?" "One can excuse a politician anything but not alcoholism," Mr Juncker added.
The rumours, Mr Juncker explained, were triggered by "a balance problem I have with my left leg, which forces me to grip the handrail when I'm on the stairs". He said the problem "goes back to a serious car accident - in 1989 I spent three weeks in a coma, then six months in a wheelchair".