At a basic level, all drugs are "harmful" in that they seek to do something to your body that is not natural. In that sense, it is poisonous. But that doesn't make drugs bad. They clearly can be very helpful in making us better. A better question to ask is if taking the drug is risky, or if the pros of taking the drug outweigh the cons. In a general sense, all medication dispensed in Singapore is strictly regulated with that in mind. Drugs are only approved for use in the treatment of medical conditions if there is sufficient scientific evidence to demonstrate they are reasonably effective with manageable side-effects. A drug is "safe" in that the pros of taking the drug to treat a condition outnumber possible cons in the form of side-effects. That said, patients typically experience varying degrees of side-effects. This is particularly the case for psychiatric medication. Some experience none, others experience many, and the list of side-effects can be very long.
To someone who experiences many side-effects which makes taking the drug unbearable, the drug may therefore seem "unsafe" if experiencing side-effects in the long-run is harmful. For example, prolonged weight problems, or insomnia, which some psychiatric medication can cause as a side-effect is probably harmful in the long run. Taking the drug might therefore not be a good idea as the risk of harm caused by side-effects could outweigh any positive benefit.
There is also some concern that taking medication is unsafe as it could be addictive in the long-run. This is tricky to understand as the term "addiction" is often used interchangeably with "dependence". The difference between the two is an addiction involves craving the drug, whereas dependence doesn't. Psychiatric drugs are not addictive in the sense you will not crave them, like nicotine in cigarettes. There, however, are more studies now showing they can lead to dependence in the sense that your body will indeed get used to having them and suddenly stopping or dropping a dose could result in discomfort. This is called "discontinuation syndrome." It is difficult to anticipate who might experience this - some can stop taking psychiatric medication with no side-effects, whereas others will suffer from a whole host of side-effects with even the slightest drop. As such, the general advice to avoid this is to see your doctor regularly if you're taking psychiatric medication and to not change doses or stop taking it suddenly.
A related question is whether psychiatric medication is effective. This is difficult to answer conclusively. There are many psychiatric conditions, many more psychiatric drugs, and even more types of patients. Unlike treating a broken bone, where what should be done is well established, treating mental health conditions involves a degree of trial and error unfortunately. My experience could be instructive. I suffer from depression, and I've gone through a long list of antidepressants. None of them really worked for me. That's not to say they didn't work for others. For every person who claimed an antidepressant did not work, you can find another who said it worked wonders for them. Anecdotes aside, scientific studies also do not offer more clarity. There are studies which show antidepressants work, and others which show they do not.
I've personally concluded that drugs do not work for me. BUT this comes after many years of trying different medication and giving each a fair shake. The best one can do is to really listen to your body and make your own decision based on your own personal experience. What's important is you try to get better and not given up hope, recognising there are many ways to tackle a mental health challenge. The use of medication is only of several possible options to get better. For me, medication did very little for me, therapy helped a little, but regular exercise helped a lot more (in terms of immediate relief).