If your child starts to use drugs you can of course forbid that or punish him, but the question is whether that helps. The child will likely use in secret and you will be even worse off. The use will happen completely beyond your perception and a conversation about drugs becomes almost impossible.
The options available to you depends on the drug, the acceptance of the drug in society and the peer pressure which your child may get in certain groups for using drugs. In a country with strict views on alcohol and narcotics, you may be able to forbid your children using drugs. In a country with less strict views, or for drugs which are accepted in society (like alcohol and tobacco in many countries) your options may be more limited. In such countries, your best option may be to try to limit the risks of choice as much as possible by discussing the difference between use and abuse. The use of substances means that you control the choice to use and only use moderately. Abuse means that you don't control the use anymore, don't keep the risks in mind and need substances to feel good.
One should note that the same drug which is misused, sometimes also is used as a medicine for specific illnesses.