England and Harlequins full-back Mike Brown says he will be "cautious" with his ongoing comeback from concussion.
He returned for the championship decider against France, but "didn't feel quite right" afterwards and missed Quins' 42-14 loss to Saracens.
"It is just not worth pushing through," said Brown.
Ireland's Jonny Sexton was stood down for three months earlier in the season and Wales wing George North will see a neurosurgeon after suffering four recent concussions.
The Northampton wing will miss Saturday's European Rugby Champions Cup quarter-final with Clermont Auvergne with his latest head injury.
Brown said he is determined to avoid an extended break, especially with the Rugby World Cup in September, but is unsure when he will return.
"With what's coming up at the end of the year, everyone's being cautious, and I'm being cautious as well," said Brown.
"After the France game I had a headache and could have easily thought 'it's Sarries, it's Wembley and I'll give it a go', but we're so clued up on it now that I don't think anyone would risk it.
"If I can do it, pulling out of England-Ireland, the biggest game of the Six Nations at that point, then I think anyone can do it, and should do it."
Harlequins play Gloucester in the Aviva Premiership at 15:00 BST on Saturday.