Personally, I don't support animal testing, this is just an essay for an English test.
Animal testing can ensure the new medicine is safe for people to use. If medicines are not tested on animals first, many people may suffer because of the _________ caused by a new medicine.
Is side effects appropriate here? But many medications have side effects.
Should I use negative effects? I think there should be some terms better than negative effects that can be used here.
I think you might find this more satisfying:
ill effects
But negative and side also work fine.
Note, you can find out what some common expressions frequently used in English by googling and reading some authentic sentences. In the old days, before the internet, we were restricted to using dictionaries and usage manuals. Now, we can go fishing for sentences, and pull up scores of good examples, in a split second!
The technical term is adverse drug reaction as outlined in this Wikipedia article Note that side effects can be beneficial although they are often thought of as purely harmful. Clinical trials also report on adverse events which happen to people taking the medication but which is not necessarily caused by it. Only if a causal link is established is it a reaction.
So your sentence would read
Animal testing can ensure the new medicine is safe for people to use. If medicines are not tested on animals first, many people may suffer because of the adverse drug reactions caused by a new medicine.
The word drug may be dropped when it is clear what is being referred to.
As an example of a side effect which turned out to be beneficial one might cite the case of sildenafil which was being trialled as a treatment for chest pains and which had a side effect on male erections. Judging by its sales when it was marketed by Pfizer as Viagra it was seen by many men as a beneficial side effect (and by Pfizer shareholders as well).
or possibly malaise or debility.
Also, I would avoid using absolute statements, such as:
...because of the _________ caused by a new medicine.
Because not all new medicines will cause negative effects.
Rather, prefer something a bit softer such as:
...because of _________ which may be caused by a new medicine.
or
...because of the potential for _________ which might be caused by a new medicine.
Complications - From Merriam-Webster:
2 - an abnormal state that disrupts a plant's or animal's normal bodily functioning.
Example: "complications set in after the surgery"
Malaise - From Merriam-Webster:
1 - the condition of not being in good health.
Example: "Her symptoms included fatigue and general malaise."
Debility - From Merriam-Webster:
1 - the quality or state of lacking physical strength or vigor.
Example: "our grandmother's debility is due in large part to her advanced age"
