Drop down list values dependant on value of another drop down list

I have a drop down box in column N, however I would like the options in that drop down box to change depending on what appears in column M.

If column M Says "Lapse" then I want one drop down box to appear in column N, If coumn M says "NTU" then I would like column N to show different drop down options.

I hope this makes sense, I am completely stumped by this and to be honest I don't think it is even possible??

Any advise would be greatly appreciated.

Thanks,