Drug charges in California resulting in convictions might have serious consequences for you. Any crime associated with drug possession, a prescription drug, drug distribution or even possession of drug paraphernalia might saddle you with unforeseen difficulties in future endeavors, such as travelling the world or pursuing higher education.
If you have done much travelling, you know that international laws regarding tourism and immigration are subject to change frequently. In fact, any country's government could create policy to restrict or deny your travel visa if you have a drug charge conviction. You can see these laws in practice in the United States visa policy towards the UK, one of the country's oldest allies, which states that any prior drug convictions might render travellers ineligible for visa-free entry. It is possible that any of the countries you would want to visit could share this disposition towards drug crime, or else take even a harsher stance.