The NHS website recommends paracetamol for chickenpox, saying there is a very small risk that ibuprofen could cause 'adverse skin reactions'.
In her post, accompanied by a picture of a Nurofen For Children carton, Miss Lyons wrote: 'Chickenpox is going round again can I please remind people NOT to give your children Nurofen/ibuprofen.
'Four different doctors from our local hospital (out of hours) prescribed it for Lewis as we couldn't get his temp down. They even administered it to him in A&E.'
When Lewis, from Warrington, Cheshire, failed to recover, she took him to Alder Hey Children's Hospital in Liverpool. Doctors there told her that ibuprofen reacts with chickenpox, making the rash go deeper.
'He ended up with septicaemia and was admitted as soon as we arrived there,' she wrote.
'Only because we persevered was he OK. This could have ended up so much worse.'
Septicaemia - also known as sepsis or blood poisoning - is called the 'silent killer' as it is hard to spot and kills up to 1,000 children in the UK each year, far more than meningitis.
Miss Lyons wrote: 'It does actually state on the Nurofen website not to take this medicine with chickenpox. But when our doctors prescribe it, who are we to question it?'
Consultant emergency paediatrician Dr Stephanie Smith, of the Royal College of Paediatrics and Child Health, said: 'Paracetamol should always be the first choice painkiller in treating children with chickenpox as we know there is a very small risk of complications [with ibuprofen].'
Children who react badly to the drug see their lesions bleed, as in Lewis's case, putting them at risk of septicaemia. However sepsis can also result from children who have not been given the drug scratching themselves.
Last night Miss Lyons - whose Facebook profile says she works in microbiology at Warrington Hospital - said Lewis had fully recovered from his ordeal last summer but still has the scars.
'My friend's daughter has chickenpox so I decided to share these photos of Lewis. I didn't expect the response to be as big as this,' she said. 'I didn't share the pictures at the time it happened because it was just so horrific. Parents need to be aware.'
The NHS Choices website says chickenpox usually clears up without treatment but, if painkillers are needed, paracetamol is preferred.
It warns that with ibuprofen and similar drugs there is a 'very small risk' of adverse skin reactions in chickenpox. Children with the virus should never be given aspirin as there is a risk of a potentially fatal condition, Reye's syndrome.
Nurofen for Children said it was looking into Lewis's case. Warrington Hospital said it had not received any complaints over Lewis but was investigating in the light of his mother's Facebook post.
