Seems like so many of them have a warning to exercise caution while driving or operating equipment as it “may cause drowsiness”. Why is this a common side effect?
In: Biology
While there are probably shared mechanisms in a lot of drugs, much of this is due to how sideeffects are counted. If you are in a drug trial, and you are given the actual drug rather than the placebo, /anything/ you experience is logged as a side effect. They look for statistically significant side numbers and if the symptom hits that number, it gets logged as a side effect. There’s often no easy way to determine whether it’s the drug or some other factor causing the side effect, so it gets logged as a potential side effect of the drug. Many people are often tired, so if they are tired while taking the drug, they’ll attribute it to the drug.
I wonder which drugs you’re talking about specifically, because a lot of drugs don’t cause drowsiness at all. In any case, many or most drugs aren’t perfectly specific in their action. What I mean is they may mimic many different messenger molecules (which are often similar to one another) in various ways. In addition to that, some drugs cross the blood-brain barrier (like benadryl), some of them don’t really cross it in any significant way (like claritin) but some will cross in ways that varies from person to person (such as zyrtec). Lastly, many receptors are intended to be acted upon by a local release of transmitter molecules (like, from an adjacent neuron). When you take a pill or receive an injection, you don’t get such a specific action.
Maintaining consciousness is a complicated neurological process and there are many pathways which affect your ability to stay awake. Some of the common pathways which can make you sleepy include GABA-ergics (alcohol and benzos like valium or xanax) and opioid receptors (morphine, heroine, codeine). Increased activity of Alpha-2 receptors (a type of adrenaline receptor) will also cause sleepiness and conversely, blocking the activity of many other histaminergic, adrenergic and dopaminergic receptors will reduce wakefulness. Many drugs will have some level of effect on these receptors whether they are meant to or not. Smaller messenger molecules especially like histamine, epinephrine (adrenaline) and dopamine are not that different from one another so drugs that try to mimic any one of those will tend to have some level of cross-reaction.
Anticholinergic effects are also often mentioned in relation to drowsiness but I think that might be a bit of a mix-up. For instance, when you overdose on benadryl the anticholinergic effect takes over and instead of getting sleepier you have confusion, sometimes agitation and other symptoms. However, CNS histaminergic receptors are well known to promote wakefulness.
I’ll admit I don’t know a five year old who could make it through this text, but I still tried to keep it simple.
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