Placebos (originally developed as an inert 'control' substance for medical trials) are faux medications 1) and procedures which researchers assumed could not possibly have any medical effect. They are now being taken seriously as treatments, after it was shown that totally inactive substances can have profoundly beneficial effects in a wide range of medical conditions.
“After four decades of probing the mechanisms of placebo responses, researchers are advancing the argument that inert pills are more than just negative controls in clinical trials: they can be a treatment in their own right.” Source Nature Vol 535, July 2016
The mechanism(s) by which they operate - which can only be psychological - are not yet explained. The implication being that many individuals, in many circumstances, somehow have the power to improve their adverse medical conditions at will (though this is normally suppressed).
Several recent experiments have examined the effects so-called Open Label placebos - that's to say medical trials in which the recipients are informed that they will be taking a completely inert substance. Even so, despite patients knowing that they're receiving a placebo, several such trials have revealed that the placebos can still have substantial beneficial effects. example
(Note that Open Label trials also exempt researchers from any ethical problems in giving inert medications to patients who are unwell.)
Extensive further resources : Program in Placebo Studies & Therapeutic Encounter (PiPS) at Harvard University, US.
As the relevance of the placebo effect becomes more obvious, it's now assumed that many medical 'treatments' to which the medical establishment can't assign any scientific basis, can nevertheless still be effective. It also throws into focus the idea that some medications which relieve symptoms might sometimes be acting via their 'placebo' routes rather than through chemical effects.
The effect is not restricted to drug treatments: increasing attention is being given to the placebo effect in surgical procedures. A 2014 paper published in the British Medical Journal found that :
“Placebo controlled trial is a powerful, feasible way of showing the efficacy of surgical procedures. The risks of adverse effects associated with the placebo are small. In half of the studies, the results provide evidence against continued use of the investigated surgical procedures. Without well designed placebo controlled trials of surgery, ineffective treatment may continue unchallenged.”
The study found that many (minor) routine surgical procedures often have little or no real medical effects, but are effective because of their 'perceived' benefits.
A similar scenario has emerged regarding recent studies looking into the efficacy of arterial stents. A 2018 article in the New York Times points to a growing number of studies which have shown that (in most cases) arterial stents to combat angina have little or medical valye - but they do help angina sufferers because of a pronounced placebo effect.
Note: An associated effect, called the 'Nocebo' effect, is given when someone reacts badly to a completely inert substance (i.e. becoming ill or made worse). As above, there is as yet no explanation as to how this might happen.