I would imagine it would depend on what you told them as well.
I am not an insurance person, but I did buy another life policy a couple of months ago. When I filled out the application, there was a long list of questions to be answered; everything from “Have you ever had a heart attack, stroke, circulatory problems?” to “Have you ever done illegal drugs?” And everything in between and sideways: family history of heart problems, for example, or family history of depression. IIRC, there were question about smoking, drinking, and illegal drug use. The same questions came out at the medical, fired at me by a nurse who wrote my answers down on her own form. But I wouldn’t be at all surprised if they cross-checked some of the answers against the blood and urine samples they took. Of course, they cannot check your blood or urine for Grandpa’s heart attack, but I think they can check them against your answers for drugs.
As for your question, elbows, as I said, I am not an insurance person, but I would think they would check with their actuaries, based on usage patterns. They do for smoking and drinking; why not for marijuana use? After establishing a usage pattern for all the applicant’s vices and everything else, they would check the actuarial tables and see if the applicant is a good risk. If so (even with marijuana use that they were told about on the form), issue the policy; if not, sorry, pal. Premiums would depend on all the factors a person can carry: age at time of application, family history of illness, job or school, married or single, vices, etc. etc. etc. It doesn’t seem to me to be simply a matter of “raising rates,” but a matter of making the premium fit with all risks and hazards in the insured’s life: drug use, tobacco use, alcohol use, skydiving, mountain climbing…
It would seem to me that the key is total honesty. If you did drugs within the past six months, and they ask, it is probably better to tell them what and how much than to lie and say “No” and have a drug test find otherwise. In the former situation, a policy may still issue; in the latter, it definitely will not. Insurers hate liars.