Avoid mixing drugs. This is the cause of many overdoses. For example, drugs like benzodiazepines can have a deadly additive effect when taken with opioids or alcohol. Additionally, taking several different drugs makes it more difficult to keep track of how much of each drug has been taken.
Test, test, test. Drug testing kits are readily available for purchase online. In the UK you can go to Reagent Tests. They can be used to determine whether a pill or powder contains any harmful drugs that may lead to an overdose.
Get clued up. Research typical doses - these are present on all the drug guides that we have. If it is legal in your country, consider investing in an electronic weighing scale.
Be aware of your tolerance. If you use a drug regularly, you will develop a tolerance to it (needing more to achieve the same effect). If you suddenly stop using it for a significant amount of time (even just a couple of weeks), your body will stop being used to it and your normal dose may suddenly become too much for you. Lower your doses accordingly.
Avoid strong drugs. The stronger the drug, the more likely the overdose. The recreational dose window becomes very small and it can be easy to go over.
Know your source. Consuming any drug bought illegally comes with risks, but does your source seem particularly unsafe or unreliable? It is always wise to take a small test dose of any drug before taking a larger dose. If it doesn't feel right, get rid of it.
Have a buddy. Having friends you trust to help you out if an accident were to happen is crucial. If you must be alone when taking a drug, consider letting a trusted friend know and determine how they can check up on you in advance.
Put your health first. Poor nutrition, unhealthy sleep habits, and overall health condition can contribute to increased risk of overdose. Find a doctor that you can be honest with and remember the importance of getting regular check-ups, especially if using substances.