"won't do any harm" is correct and usable in written English.
You can use this phrase when you want to reassure someone that an action is safe or beneficial. For example: "You can try giving him just one small piece of candy - it won't do any harm."
An improved mental focus won't do any harm.
"It certainly won't do any harm in that scenario.
It won't do any harm to a child who is already immune to have a dose of MMR".
It certainly won't do any harm, but it falls a long way short of being a full answer.
"In such a second-order election, it certainly won't do any harm," says Jamie Barclay, who runs the operation in Britain.
And a reputation as a chap who can sell businesses for remarkable prices – Argos and Arcadia being prime examples – won't do any harm.
In the same way as young people may come to like alcohol, tobacco and coffee (all of which initially taste nasty, but deliver a pleasurable chemical kick), chili-eating normally starts off as a social habit, bolstered by what Mr Rozin calls "benign masochism": doing something painful and seemingly dangerous, in the knowledge that it won't do any permanent harm.
I was lucky to be able to make arrangements to stay with friends who live near the hospital where I get my treatment and to begin my chemotherapy cycle just two days late, a delay that won't do any lasting harm to my health or my treatment plan.
[Okay, we're impressed by rotgutwhiskyandsoda] rotgutwhiskyandsoda 07 March 2014 10 09am re the 6 months out of date veg suet, I wouldn't use it oils tend to oxidise in contact with the air, developing off flavours ( tho probably won't do any great harm ).
Now, if you have a hungry baby crawling around and notice she has eaten a leaf, don't be alarmed: They won't do any real harm to humans.
