My issue is a bit unusual. I've passed a medical exam in Singapore for my company. They tested me positive to drug cannabis but never caught me as I left the country before result issued. Hospital send notification to minister of health MOH which has sent report to Central Narcotics Bureau with all my details. Do I have to avoid going to Singapore for my entire life?
Good move getting out of Singapore in time, since simple possession of a controlled substance in Singapore (including cannabis metabolites in your system) is punishable by up to 10 years in jail. First time offenders usually get away with "only" 6 months in the Drug Rehabilitation Centre, which is located within Changi Prison and by most accounts isn't much better than actual jail.
The bad news is that Singapore inherits most of its legal code from the UK, including the globally rare concept of having no statute of limitations for criminal offences. So, yes, I would strongly recommend never setting foot in Singapore again, even in transit. Sorry.
Update: While Singapore's laws are clear that consuming drugs outside Singapore is an offense if you are a local citizen or permanent resident (Sec. 8A), it's less clear what happens if you are not one. This implies that saying you failed the test because you consumed drugs overseas may be a defense, although the burden would still be on you to prove that this was the case.
In addition to staying away from Singapore, the one topic that hasn't been touched is extradition.
Most countries with extradition agreements with Singapore only cover things seen as major crimes, and a marijuana offence would not fall under that category, but Malaysia is an example where they will extradite for anything carrying more than a 6-month sentence, so it's possible if you went to Malaysia you could be apprehended and taken back to Singapore. This would require that you actually be charged with a crime in Singapore and it's not clear from your question that this has actually happened yet.
A lawyer would not be a bad idea if you believe you have been charged with a crime in Singapore and wish to travel to other countries where you may face threat of extradition. We also don't know your home country, so this would also apply if your home country has permissive extradition arrangements with Singapore.
Hire a Singaporean lawyer experienced in drug offences and ideally migration, and ask them what their advice is. I don't think this website is a good place to seek legal advice over something so serious!
Edit: I suggest you contact a lawyer from outside Singapore, and recommend that you do not enter Singapore unless you are advised that you will be able to without risk of arrest.