This drug concentration calculator is an easy tool to determine the amount of substance left in your system. It does so by looking at the elimination half life of a medication. Use it to keep constant levels of medicine in the body, or avoid unwanted interactions between substances. Try a demo to get an idea of how it works. Or enter your data as follows: Drug - drug name on the graph. It can be anything, but half life for common drugs is auto-filled. Dose - amount of the drug taken. Half life - half life of medicine. Select a drug from the list of suggestions to fill it out automatically. Offset (optional) - how long after the initial administration you took another dose. You can add unlimited ingestions to track multiple drugs or repeating doses of the same one. Once you enter the required data for at least one ingestion, the graph shows the results: Time - how much time has passed since you've first taken the drug. Residuals - how much drug is left in your body. Hover on the graph to get a precise amount of drug left at any time. You can consider the drug eliminated when about 97% of it is gone. That's when most drugs no longer have any effect.