Abnormal heartbeat from Diphenhydramine

Abnormal heartbeat, clinically termed arrhythmia or dysrhythmia, refers to any disruption in the normal electrical conduction system of the heart that produces an irregular, abnormally fast, or abnormally slow heart rhythm. The heart's electrical system normally maintains a precise, coordinated sequence of contractions, and substances that interfere with the ion channels, autonomic innervation, or conduction pathways governing this system can produce rhythmic disturbances ranging from benign palpitations to life-threatening cardiac events.

The mechanisms of substance-induced arrhythmias are diverse and substance-specific. Stimulants (cocaine, amphetamines, MDMA) produce arrhythmias primarily through catecholamine excess — flooding the heart with norepinephrine and epinephrine, which increases automaticity (the tendency of cardiac cells to fire spontaneously) and can trigger ectopic beats, tachycardia, or more dangerous rhythms. Cocaine additionally blocks cardiac sodium channels, a mechanism that can produce ventricular arrhythmias similar to those seen with local anesthetic toxicity.Drugs that prolong the QT interval (certain opioids including methadone, some antihistamines, various research chemicals) increase the risk of torsades de pointes, a specific and potentially fatal ventricular arrhythmia.Electrolyte depletion from prolonged stimulant use, dancing, sweating, and inadequate nutrition disrupts the ionic gradients that cardiac cells depend on for normal electrical activity.

The subjective experience of arrhythmia varies widely. Benign palpitations — the sensation of the heart "skipping a beat" or producing an occasional extra-strong beat — are common, usually harmless, and typically caused by premature ventricular or atrial contractions that most people experience occasionally even without substance use. However, sustained rapid rhythms, sustained irregular rhythms, the sensation of the heart "fluttering" rapidly, or episodes accompanied by lightheadedness, chest pain, or shortness of breath represent potentially serious cardiac events that warrant immediate medical evaluation.

Harm reduction note: Abnormal heartbeat is one of the effects that should always be taken seriously. If you experience sustained rapid heart rate (above 150 bpm at rest), a heart rate that remains irregular for more than a few minutes, chest pain, or feelings of faintness during substance use, seek medical attention. Do not attempt to "wait it out" with potentially dangerous arrhythmias. Stimulant users should be aware that redosing, combining stimulants, dehydration, electrolyte depletion, and pre-existing cardiac conditions all substantially increase arrhythmia risk. Individuals with known heart conditions, long QT syndrome, or a family history of sudden cardiac death should avoid stimulants and other cardiovascular-active substances entirely.