You're drawing a line that doesn't exist. By all means, alcohol is an addictive hard drug with withdrawals that can actually kill or cause long-term brain damage. The only other class of drugs that have the same dangerous withdrawal are benzodiazepines, which target the GABA system. Alcohol is dirty in comparison, touching your GABAergic, serotonergic, dopaminergic, opioid and glutamate systems in your brain. You can overdose and die from consuming too much alcohol, which is not the case if you consumed too much marijuana or benzodiazepines (by themselves).

Of course, if we define moderate use of a substance as the level of consumption where health effects are little to none, you can consume something without jeopardizing your health.

You seem to be implying that either heroin or cocaine cannot be consumed in moderation (which is false[0][1]), or that their moderate consumption is tremendously bad for your health. If it's the latter, prescription opioids and stimulants like methylphenidate can be taken in moderation with limited health effects.

Many prescription opioids are very chemically similar to heroin. All prescription opioids work primarily via the same mechanism as heroin. Heroin is used in a medical context under the name diamorphine.

[0] http://jod.sagepub.com/content/13/2/219.abstract [1] http://www.doctordeluca.com/Library/AbstinenceHR/ControlledH...