Is writing information about treatments for diseases on the internet—with a warning against using it without consulting a doctor—considered a sin, or is it better not to record it to avoid harm, and is that considered concealing knowledge?

There is no harm in disseminating medical information to benefit medical students and others. Indeed, the one who does so is a doer of good. It is sufficient to warn against taking medication except with a doctor's consultation. Not recording it does not fall under the sinful concealment of knowledge.