"Good in Bed" has given an excellent response. In addition, behaviorally, alcohol and drugs very often produce uncontrolled and embarrassing behavior that your intended sexual partner will not find attractive at all. You may find yourself alone at the end of the night and not even know why or remember the evening at all. You won't have to worry about the fact that you can't breathe during sex because you can't process enough oxygen for that level of exertion. You won't have to worry about the impact of doing drugs on your DNA and therefore your offspring. You won't be aware of the fact that you are constantly disappointing your partner because you are out cold half way through sex and therefore your relationship is tenuous at best. You won't have to worry about any of that with this partner at least because s/he will find someone else who doesn't get drunk and get high and you will be alone. The downward spiral from there is pretty bleak. Bottom line, sexual health notwithstanding, drugs and alcohol to excess will ruin your life and health faster than you can figure out what 'to excess' is.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.