Other messengers: histamine, opioids, hormones and more
Rounds out the messengers you'll meet as side effects and drug targets: histamine, opioid peptides, hormones, endocannabinoids and adenosine.
Where this fitsConnects to 2 · Unlocks 1
- Connects to
- Primary effects and side effects — Antihistamine drowsiness is a classic predictable side effect.
- Acetylcholine: nicotinic, muscarinic and AChE — Diphenhydramine also blocks muscarinic receptors.
- Unlocks
- Absorption and routes of administration — Opioid examples reappear: fentanyl patches, naloxone, buprenorphine/naloxone.
Walk through
Histamine and drowsiness
First-generation antihistamines (like diphenhydramine/Benadryl) block H1 receptors in the brain → drowsiness. Second-generation (Claritin, Zyrtec) don't act at brain H1 receptors → no drowsiness.
Analogy and worked example
Analogy: A master key that fits two locks: diphenhydramine opens a sleepy door (H1 block) and a confusing door (muscarinic block) at the same time. Second-generation antihistamines are cut so they can't get into the building (the brain) at all.
Why does Benadryl make you sleepy but Claritin doesn't?
- Both block H1 receptors to relieve allergies.
Tip: the ← and → keys move through the lesson.
Full lesson: Histamine, peptides and other messengers