This subject is extensive but we will take a brief look at the overall broad picture with regard to how it might affect our approaches to self defense, and perhaps more so, to have even more good reasons to avoid situations whenever possible. Much of this of course requires good use of spacial awareness as a prerequisite.
Mood influencing compounds are many, including many illegal drugs and also those prescribed (over prescribed?) for supposed medical conditions. It's worth looking at these with regard to how an assailant encounter might be experienced and how their behavior can be anything from bizarre to downright dangerous.
First, we are all uncomfortably used to 'recreational' drugs in society and all their many effects, which of course because of dependence result in crime simply for financing. That in itself can lead to assaults on the person with muggings and sometimes homicides in order to rob and steal. Furthermore and perhaps of equal or even greater importance are the psychotic effects surrounding drug use, promoting acts of violence.
The list of drugs in this category includes such popular offerings such as opiates (Percs, Oxy, Heroin, fentanyl etc), PCP (Phencyclidine), Ketamine, Crystal Meth, Cocaine (and derivatives), Ecstacy, and so on... it's a long list. This category of drugs will for the most part present with the most extreme effects, such as paranoia and aggression with a risk of unprovoked attack.
Perhaps in passing, it might be worth reminding that alcohol is itself a potential mood altering substance and in many instances can also change a mild mannered individual into an aggressive or even paranoid threat. A drunk may have a compromised motor system and be unsteady but nevertheless be capable of dangerous attack actions.This usually at least may be readily determined.
Second, the prescribed pharmaceuticals within which group we find what might be regarded as potentially more subtle effects, but sometimes no less dangerous. Like with 'recreational' drugs, again the list is long but the following describes some examples, many of which supposedly treat depression or anxiety disorders. From WebMD:
Depression -
Anxiety
Now, let's look at what might be seen as the most important considerations, particularly with regard to prescribed pharmaceuticals. Opinion varies considerably when it comes to actual need for many of these such that one camp considers over-use as being all too common. Take supposed ADHD treatments dished out to young people as one example, plus depression and anxiety medications being too readily administered when patients could be treated by attention to life style aspects etc. It might be said that the 'easy option' so often is to pull out the prescription pad and quickly move on to the next patient - a cynical view maybe but one that is often cited.
Unwanted effects, whether from 'recreational' drugs or prescription medications can often be conveniently lumped together, even if separated by degree. A potential antagonist 'under the influence' might display anything from unsteadiness and irritability to plain and paranoid type aggression. This may usually be easily deduced early in an encounter.
Perhaps of more importance to consider is the possibility of withdrawal effects - either the 'druggie' who is missing a fix, or the prescription patient who has suddenly stopped a psychoactive drug when controlled cessation should be mandatory, although some patients can exhibit unwanted side effects even during their course of treatment - enough to provide anti-social and even dangerous behaviors.
The bottom line, and the main purpose of this treatise, is to help increase awareness of factors that might be encountered in a threat situation. A 'rational' criminal might be sufficiently in control to not be considering harm if achieving their main purpose with no resistance - perhaps a case where the fit victim can escape without need for significant action. On the other hand, the criminal who is under various effects from 'substances' might be considering inflicting bodily harm regardless of a victim's aquiescence - a potentially classic case where defensive action may be the only resort.
Judgement of a situation may never be easy but some tell-tails might include noticing dilated pupils, general agitation and aggressive gestures coupled with the 'spidey sense' we should all learn to recognize. Whether time and circumstance permit, the decision to employ less-than-lethal means (OC spray) or the need to resort to lethal force with a firearm will depend on an individual's ability to rapidly assess the threat level.
In closing, it might be mentioned that there have been numerous cases where parents (or even just friends) of substance affected offspring say little or nothing at a stage where preventative action could be taken to reduce the possibility of adverse actions. Sadly these days, it seems to be teens leading the crime figures.