There are two ways in which apparently inert pills could be harmful - (1) by getting in the way of real medicine and (2) turning out not to be inert after all, and one way (3) in which inert pills are harmless to people who didn't need a real medicine in the first place. I hope this post below is clear in explaining this in more depth but let me know in the comments if bits need tightening up, thanks.
Recently an article with the same title as this blog post was published on an alternative health news website which expressed amazement that this could possibly be true.
"The two dominant criticisms made against homeopathic remedies are antithetical to each other. Critics complain that due to a lack of regulation homeopathic remedies are dangerous, but then they turn around and say that homeopathic remedies are simply useless sugar pills with no therapeutic effect whatsoever, and no better than a placebo.Both can be true. In fact there are two separate ways in which homeopathy can be dangerous and yet still be a harmless placebo, here's how.
Both cannot be true." From http://www.donotlink.com/ezid
(1) Imagine you're in a field and there's a beautiful hand-made wooden gate between the field and the road. Suddenly a bull appears but you can't get this lovely gate open in time and things don't go too well with you bull-wise. The inert and harmless gate has impeded you in getting the help you need.
(2) Imagine you're in the same field, there's no bull this time but on opening the gate you discover that it's been badly made and you get a nasty splinter from using it.
(3) This time you're in the field, there's a nice-looking gate and no bull about but you use the gate with no problems and leave safely.
Hopefully you can see where I'm going with this.
In situation (1) you're in peril and need appropriate help. The inertness of homeopathy's sugar pills can't help you here unfortunately. When someone needs appropriate medical attention seeing a homeopath or faffing about with homeopathy pills instead might delay the correct treatment. While many homeopaths are perfectly sensible and work appropriately with medical doctors enough of them don't to cause problems to patients and confusion to the public. There are far too many homeopaths claiming they can help with some serious conditions (asthma, diabetes, high blood pressure) and plenty of them suggesting homeopathy as alternatives to vaccines. It's these homeopaths that have caused skeptics to pay particular attention to homeopathy as a whole.
An absence of treatment (ie empty homeopathy pills) where treatment is actually needed could be dangerous. However an absence of treatment, where none is needed, is not likely to be harmful and as homeopathy contains such highly diluted amounts of substance - if properly prepared - there's unlikely to be anything there that can cause any active harm - this is basically situation (3).
However - and this is important, some 'low potency' homeopathy pills are not diluted as much as people might think, and may still contain some of the active ingredient. It's possible for these, sold over the counter without prescription, to cause people harm and this is situation (2). This testimony to the FDA (Food and Drug Administration, USA) from Adriane Fugh‐Berman is worth a read, see in particular the section on 'Safety is not a given'.
Situation (2) also arises where poorer manufacturing processes, or deliberate 'doping' with an active ingredient, results in pills being sold as homeopathy but which actually contain something that shouldn't be there. It is this class of pills which appears to have caused the FDA to sit up and take note. Pills that contain things that they shouldn't (especially if the ingredients aren't properly listed or the risks associated with them explained) could be dangerous and cause active harm to people taking them.