The NHS is obliged from today to make the drug available for all eligible patients after announcing the change in guidelines three months ago.
Dr Robert Niven, senior lecturer in respiratory medicine at the University Hospital of South Manchester, said: "Many patients unnecessarily accept the everyday limitations and sudden asthma attacks, which are a feature of severe allergic asthma.
"I would urge anyone who has been on numerous courses of oral steroids, for treating their asthma, be seen by their doctor for an assessment."
About 14,315 people in the UK are thought to suffer from severe allergic asthma, which is a life-threatening condition.
Experts warned that the treatment would not be suitable for all patients, but said that it would provide "a massive improvement" for others.
Dr Samantha Walker, Deputy Chief Executive of Asthma UK, said: "Without it, many [patients] would be virtually housebound because of breathlessness and living in constant fear of the next life-threatening asthma attack."
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