Like Dot Cotton in EastEnders, I am finding it hard to quit. As the ban on smoking in enclosed public places comes into force in England, hundreds of thousands of nicotine addicts like us will try to stop.

The Department of Health has set aside £112m for this year and next for the NHS Stop Smoking Service which runs a national network of clinics providing support and advice on aids to giving up, such as nicotine replacement therapy and the prescription-only, nicotine-free drug bupropion hydrochloride, trade name Zyban.

Another anti-smoking drug varenicline, known as Champix, has just been given draft approval by the National Institute for Health and Clinical Excellence.

In a recent storyline, although Dot publicly refuses all offers of aids to quit, she almost passes out, with a look of ecstasy on her face, after a long, deep inhalation of nicotine from a nasal spray.

Nicotine replacement therapy is not for me. All the products - gum, patches, lozenges and microtabs - are addictive, although they don't cause cancer. Ex-smoker friends are still chewing gum and taking the lozenges years after stopping.

I decided to try Zyban which had worked for two friends. But when I asked my north London GP for a prescription she said the practice had a policy not to prescribe Zyban. Why? First she said it was not clinically proven to be effective, though there have been studies in the US and the UK showing Zyban is an effective aid to quitting when accompanied by counselling. The drug affects the brain's chemistry in a way similar to nicotine and is supposed to reduce the craving and withdrawal symptoms.

A Department of Health guidance paper in February 2007 said: "A smoker who tries to quit with the NHS Stop Smoking Service and NRT/Zyban is up to four times as likely to succeed than by willpower alone."

Asked what other reasons the surgery had for refusing to prescribe Zyban, my GP cited side effects such as headaches, high blood pressure, difficulty sleeping, seizures and even psychotic behaviour; patients would have to be monitored regularly. Maybe they didn't want hundreds of ex-smokers needing regular check-ups and counselling?

So where could I get this drug, available on the NHS since 2000? I was referred to an NHS Stop Smoking clinic at the Royal London Hospital in Whitechapel, east London, funded by a different primary care trust, a half-hour Tube ride away. After filling in a four-page questionnaire and a pep talk from a Stop Smoking adviser (now my "buddy", who I will see every week for support), I received my prescription. Puzzled why my surgery refused to prescribe Zyban, she said: "It's available in some areas and not others. It's a postcode lottery."

As I write, I have been taking Zyban for two weeks. For the first week you take a tablet a day and smoke as normal. You set your "quit day" then take two tablets a day for the next eight weeks. I am on day seven. The first two days of not smoking I was agitated. But by 4am on day three, when presumably the Zyban had worn off, I had strong physical cravings.

I cracked on day four, while with friends in a smoky bar. I am now once more a determined - sort of - non-smoker. But, I worry, for how long? A UK study found only 15 per cent of people who quit using NRT or Zyban remain smoke-free after 52 weeks.

As I write, Dot is still blissfully smoking. But when she and I are unable to smoke in restaurants, bars and pubs like the Queen Vic, I for one will be relieved.