(CNN) European Commission President Jean-Claude Juncker was suffering from a "painful attack of sciatica" at a NATO gala Wednesday that caused him to stumble, according to a spokesman.

Juncker was not drunk, the spokesman said, when he was seen on video unsteady on his feet before a dinner of NATO leaders in Brussels.

Unbalanced and struggling to walk, he is seen being helped down some steps by other attendees. He was later seen entering the dinner venue in a wheelchair.

Speaking Friday, European Commission spokesman Margaritis Schinas said that Juncker "suffered from a particularly painful attack of sciatica, accompanied by cramps" that affected his ability to walk, but soon passed.

Juncker attends the second day of the NATO summit following the incident Wednesday evening.

Asked if Juncker was drunk at the time of the incident -- allegations he has faced before -- Schinas said it was "more than tasteless that some press tried to make insulting headlines by exploiting President Juncker's pain." He added: "I don't think this is elegant, I don't think this is fair."

He did not answer a later question regarding whether Juncker had had an alcoholic drink before the dinner.

Schinas confirmed that the president attended the dinner as planned and completed a full schedule of events the next day. Schinas said that he was not aware that Juncker received any medical treatment on Wednesday evening, but said that the president does take medication for his condition.

Asked to confirm that Juncker's condition was not preventing him from carrying out the functions of his role, Schinas said, "yes, I can confirm that."

Juncker, who was elected to the office in 2014 for a five-year term, has faced allegations of drunkenness on a number of previous occasions, appearing unbalanced and displaying erratic behavior during public appearances.

Most recently, on June 21, he appeared to struggle while walking down the stairs in the lower house of the Irish parliament, en route to the podium to give a speech. Before beginning his planned remarks, Juncker explained: "I have some difficulties to walk. I am not drunk; I have sciatica. I would prefer to be drunk."

Juncker has a packed schedule next week, with trips to Beijing, Tokyo and Madrid in the diary. Schinas confirmed Friday that the president would be completing his "very demanding agenda" and that there were no plans for him to be accompanied by a doctor.