they already do have such a pill... it's called marinol... and it's highly, HIGHLY unpredictable.... you can take the same dose under the same conditions, one time it gets you so high, you end up sick, the next you'll be 100% sober (notice i said same dose and conditions...)

it's extremely unreliable, and it's also extremely expensive... a bottle of 30 if i remember right costs atleast a few hundred.... and it's only given for a few select things.... AIDS/cancer patients making up the majority...