Key facts & rules

• Depressants: slow nervous system (e.g. alcohol, heroin) — reduce reaction time.
• Stimulants: speed up nervous system (e.g. caffeine, nicotine, cocaine).
• Alcohol: damages liver (cirrhosis), brain, increases accident risk.
• Nicotine in tobacco: addictive; causes increased heart rate and blood pressure.
• Tar in tobacco: damages cilia, causes lung cancer and chronic bronchitis.
• Carbon monoxide in tobacco: binds to haemoglobin, reduces O₂ transport.
• Antibiotics: kill bacteria by destroying cell walls or inhibiting protein synthesis. Not effective against viruses.
• Antibiotic resistance: bacteria survive antibiotic treatment through natural selection → resistance spreads.