Unfortunately all medicines have side effects, it just depends on the amount (or dose) which is used. When a drug reaches clinical testing a lot of research is carried out to try to find the right dose to use in clinical practice. This usually involves healthy volunteers who take the drug in very controlled conditions at increasing doses to look for any side effects. If there are too many side effects or they are more serious than predicted in pre-clinical trials the medicine may not get approved for use on patients.
I think Lori-An’s answer is good. I’d also add that it depends on how serious the illness is that the medicine is meant to cure or treat. Because if the illness is very likely to kill a person like cancer then people are more likely to put up with more serious side effects. So if the side effects are not worse than the disease then a medicine might be allowed to be used on patients. But if the side effects will cause more problems and make the person more ill then it probably wouldn’t be approved.
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Michelle commented on :
I think Lori-An’s answer is good. I’d also add that it depends on how serious the illness is that the medicine is meant to cure or treat. Because if the illness is very likely to kill a person like cancer then people are more likely to put up with more serious side effects. So if the side effects are not worse than the disease then a medicine might be allowed to be used on patients. But if the side effects will cause more problems and make the person more ill then it probably wouldn’t be approved.