The link between smoking and multiple sclerosis (MS) is "clearer than ever", with those who smoke more likely to develop the condition and become disabled more quickly, a charity has warned.
The MS Society said it has completed a major evidence review into the connection between smoking and the chronic lifelong and disabling condition, which affects the brain and spinal cord and has no cure.
One study found that quitting smoking could delay the onset of secondary progressive MS - a form of the condition that has no treatment - by as much as eight years.
Its research also found most people with MS do not realise the connection with smoking - despite the fact official guidance advises healthcare professionals to tell people as soon as they are diagnosed.
In a recent study, the majority of people with MS (89%) did not know anything about the risks of smoking and MS.
Ahead of October's annual Stoptober campaign, the charity is warning that smoking can make MS more active, and worsen and speed up the accumulation of disability.
"Looking at all the evidence, it's clear smoking can make MS worse and harder for the brain to fight the condition," Dr Susan Kohlhaas, the charity's director of research, said.
"Over 100,000 people in the UK have MS and, in light of this review, we are encouraging and supporting every one of them who smokes to quit - it could make a difference to how their MS progresses.
"It's not just people who have MS who need to be aware of this though, as people who smoke are more likely to develop MS than people who don't.
"It can be hard to give up, but Stoptober is a great time to quit because of the support of thousands of others doing the same thing."
Management accountant Tamar Packford, 43, from Blackpool, was diagnosed with relapsing MS four years ago.
She said: "I've been smoking about 20 cigarettes a day since I was 16, but had no idea it could be making my condition worse.
"Obviously everyone knows cigarettes are bad for you, but I think very few people realise it might affect MS symptoms, or make MS progress faster.
"It's frightening but, if quitting could keep me out (of) a wheelchair longer, I'm thinking very differently now and definitely considering giving up with some support from the NHS and Stoptober."
Research suggests smoking can cause further damage to the myelin sheath - the protective layer that surrounds the nerves, which is affected in people with MS.
This prevents messages getting through properly, causing common symptoms such as vision, mobility and cognitive problems.
Research also shows an association between smoking and the number and/or size of brain lesions appearing in MRI scans.
This increased damage could be the reason people with MS have less ability to fight the condition, or experience worse symptoms earlier.
Smoking can also impact how effective treatments are, meaning more relapses.
Dr Waqar Rashid, consultant neurologist at St George's Hospital in London, said people with the condition should be given clearer information about the link with smoking.
"MS can be painful and unpredictable, and is often stressful to manage," he said.
"Some people with MS believe smoking helps them manage stress, and healthcare professionals can be reluctant to take that ally away from someone who's just been diagnosed.
"But knowing that continuing to smoke might impact the disease and its progression could make a radical difference to some people.
"MS specialists must make sure these conversations are happening as soon as is appropriate, and make it a routine part of their MS consultations."
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