1. All medications have effects — usually multiple ones. The hope (and hopefully confirmed by testing) is that the effect you want ("Make me not be sick with this disease!") is more valuable than the "side" effects you don't want (tumors, cancer, hair loss, dementia).
2. The idea of a perfectly safe medicine for all people in all circumstances is a myth promulgated by drug companies and people with an unclear understanding of what it means to introduce something into the body (see #1).
3. The risk of most of those effects is, usually, very small, but you need to know about them anyway (esp. the ones that are increased when other conditions, like other drugs you're taking, come into play).
That having been said, the FDA is not always the best about requriing clear and definitive testing of drugs (vs. accusations that the FDA is too slow in approving drugs that could save lives, so name your poison), and it's not always the fastest at admitting it might have let something through previously that new evidence indicates is causing peoples' spleens to explode (cf. Prescott Pharmaceuticals).
And then you get drugs that have known "effects" but which are now being prescribed for other "effects". Classic here is scopolamine, which was originally used as a med to give to women in labor because it didn't knock them out, it just caused them to forget their pain as fast as they had it (this, remarkably, did not make the women in question any calmer), but is now used as a sea/motion-sickness medicine ….
… which, in some people (like my first wife) still causes (imagine that) short-term memory impairment. Fun times!
Reshared post from +Cracked.com
Turns out Lipitor’s not only good for lowering cholesterol, it can also help clear your brain of boring old memories like “who you are” and “where you live.” #SideEffects
The 5 Most Horrifying Side Effects of Common Medications
If you want to terrify yourself, go into your medicine cabinet and read all of the really weird side effects at the bottom of the label.