Marijuana's Effects on the Brain, Heart, Lungs & Reproduction: Evidence-Based Overview

Cannabis and the Endocannabinoid System

Δ⁹-THC activates CB1 (brain, peripheral) and CB2 (immune) receptors, modulating neurotransmitter release. Effects are dose-, route-, and user-dependent.

Neurological & Psychiatric Effects

• Acute: Impaired short-term memory, attention, coordination; euphoria/anxiety; psychosis (high THC, vulnerable individuals).
• Chronic heavy use (adolescent onset): Associated with ↓ IQ, altered brain structure (hippocampus, prefrontal cortex), ↑ schizophrenia risk (OR 2–4 in meta-analyses). Causality debated; genetic confounding likely.
• Cannabis Use Disorder (CUD): ~10% of users; 17% if started in teens. Withdrawal: irritability, sleep disturbance, anxiety (DSM-5).

Cardiovascular Effects

• Tachycardia (+20–50 bpm), ↑ cardiac output, postural hypotension.
• Acute MI/stroke risk ↑ 4.8× in first hour post-use (case-crossover studies); mechanism: catecholamine surge, endothelial dysfunction, plaque rupture.
• Contraindicated in established coronary disease.

Respiratory Effects (Smoked)

• Bronchitis symptoms (cough, sputum, wheeze) dose-dependent; improve with cessation.
• No clear ↑ lung cancer risk in never-tobacco smokers (pooled analyses), but molecular changes (p53, Ki-67) suggest potential.
• Vaping: EVALI risk from vitamin E acetate; long-term safety unknown.

Reproductive Effects

• Men: ↓ sperm count, motility, morphology; ↑ aneuploidy; testosterone suppression (reversible).
• Women: Disrupted ovulation, luteal phase defects; THC crosses placenta → ↓ birth weight, ↑ NICU admission; excreted in breast milk.

Other Systems

• GI: Hyperemesis syndrome (cyclic vomiting, compulsive hot bathing) in chronic users.
• Metabolic: Acute ↑ appetite ("munchies"); paradoxically, epidemiological ↓ obesity/DM2 prevalence (mechanism unclear).
• Immunomodulation: CB2 activation suppresses cytokine release; clinical significance uncertain.

Harm Reduction

• Delay onset until ≥25 years.
• Choose lower-THC, higher-CBD products.
• Avoid combustion (use vaporizers, edibles with known dosing).
• Don't drive ≥4 hours post-inhalation, ≥8 hours post-edible.
• Screen for CUD in primary care.