18．The health dangers of smoking have been well reported．For years，doctors have warned that smoking can cause lung cancer．Studies also have linked smoking to several other kinds of cancer，heart attacks，and a number of other medical problems．The repeated health warnings have had an effect．Thousands of people have stopped smoking．But as many people have learned，stopping smoking can be very difficult．
A major problem apparently is the nicotine，a poisonous substance found in tobacco．Small amounts of nicotine are released as tobacco is burned．And over many years of smoking，a person's body becomes dependent on the substance．An attempt to stop smoking brings the problems of nicotine withdrawal（脱瘾过程）．
American medical researchers have found that nicotine withdrawal causes people to become angry very easily，aggressive，worried and unable to think sharply．Many people who have tried to stop smoking would confirm those findings．
To help people give up smoking，some tobacco companies have proposed chewing tobacco，or using snuff as safe substitutes（替代物）．Snuff is a tobacco powder that usually is breathed in．But many people put it between their gum and cheek．
A group of British scientists said recently that many lives could be saved by a major program to get cigarette smokers to change to snuff．An American scientist，Allan Blum，disagreed．Dr．Blum argued that snuff，put in the mouth，can cause gum disease，teeth problems and throat problems that lead to cancer．Dr．Blum also warned against chewing tobacco as a substitute for smoking．He said chewing tobacco can lead to cancer of the mouth，throat or digestive system．
Medical researchers say another possible substitute for smoking is a special nicotine chewing gum．The gum is already sold in Europe and Canada．A report says research shows that the chewing gum will increase the amount of nicotine in a person's blood．This helps satisfy some people's desire for a cigarette．However，the report says，the chewing gum also leads to some unpleasant side effects．And it may be dangerous for people with heart problems．
Two other American scientists say they have found that the most effective way to stop smoking is to do it completely，suddenly，all at once．The report by Saul Shirtman and Dr．Murray Jogbett says that by stopping all at once，the worst problems of nicotine withdrawal are over in just a few days．But，they say，by continuing to smoke，even a little less each day，the problems of withdrawal never end completely．
| Bad effects of smoking | Many kinds of disease and a number of other medical （61）problems are connected with smoking． |
| Difficulty in stopping smoking | The （62）reason for this is that smokers are （63）
dependent on nicotine，a poisonous substance in tobacco，and people （64）trying/attempting to stop smoking have emotional problems and are unable to think sharply． |
（65）
Ways/Measures to stop smoking | Looking for substitutes for smoking | Snuff | Causing gum disease，teeth problems and throat problems （66）leadingto cancer． |
| Chewing tobacco | Causing cancer of the mouth，throat or digestive system |
| Nicotine chewing gum | ◆Helping satisfy some people's desire for a cigarette by （67）increasing/raisingthe amount of nicotine in one's blood．
◆Having some side effects and being dangerous for people who （68）sufferfrom heart problems． |
Giving up smoking （69）
completely，suddenly，all at once | In a few days，the worst problems of nicotine withdrawal no longer （70）
exist/appear． |
