Here are the best word choices for the provided sentences:
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Secondly, marijuana has been proven to be responsible for a number of - physical defects.
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Most young people today are likely to come across a joint being passed around at some point, and more than half will accept a drag of what many consider to be a harmless substance.
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However, although occasional use of the good old weed is probably no cause for concern, consistent long-term use has been known to cause physical problems such as “impotence and erectile dysfunction, raised testosterone levels in women and an impaired immune system” (Els, 2016: 17).
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On a greater level, life-threatening diseases associated with marijuana use are lung cancer, emphysema, heart attack, and brain damage.
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However, contrary to popular belief, marijuana can cause a number of physical problems.
So, the filled-in paragraph would read:
"Secondly, marijuana has been proven to be responsible for a number of physical defects. Most young people today are likely to come across a joint being passed around at some point, and more than half will accept a drag of what many consider to be a harmless substance. However, although occasional use of the good old weed is probably no cause for concern, consistent long-term use has been known to cause physical problems such as “impotence and erectile dysfunction, raised testosterone levels in women and an impaired immune system” (Els, 2016: 17). On a greater level, life-threatening diseases associated with marijuana use are lung cancer, emphysema, heart attack, and brain damage. However, contrary to popular belief, marijuana can cause a number of physical problems."