Correlations between usage of 18 legal and illegal drugs: alcohol, amphetamines, amyl nitrite, benzodiazepine, cannabis, chocolate, cocaine, caffeine, crack, ecstasy, heroin, ketamine, legal highs, LSD, methadone, magic mushrooms (MMushrooms), nicotine and volatile substance abuse (VSA). Links indicate correlations with absolute values of Pearson correlation coefficient |r|≥0.4. The correlation are considered weak if 0.4>|r| (no lines); medium if 0.45>|r|≥0.4 (blue dashed lines); strong if 0.5>|r|≥0.45 (orange dashed lines); and very strong if |r|≥0.5 (green solid lines)