"Toxin X is a substance that will make you violently ill for a few hours. However, it has no long term effects of any kind. As an experiment in psychology, I'm offering you a million dollars if tonight at midnight you fully intend to drink toxin X by tomorrow at noon. You don't actually have to drink the toxin; all you have to do is to intend to drink it. Your intention will be tested by a device similar to a polygraph which my people have developed and which has been shown to be 100% accurate. If at midnight you have the intention, a million will be wired to your bank account. The only other conditions are that you are to make no bets, do anything that will cause you to become irrational, or arrange for any way to avoid the effects of the toxin."

Suppose you decide that being ill for one day is a reasonable price to pay for a million dollars. Your first thought is to therefore agree to the proposal. It then occurs to you that you won't even have to become sick in order to win the money. All you have to do is to intend to drink the toxin. You don't actually have to carry out your intention.

But now if you know ahead of time that you don't actually have to drink the toxin, then you can't really intend to drink it. So you tell yourself you really do have to drink it. But then if at midnight you really did intend to drink the toxin, and you got the million, then come the next day you would no longer have any reason to drink it: you've already been paid and drinking the toxin would make you unnecessarily sick.

Is there any way for you to win the money?

I didn't read all the comments below, but here's a solution that sort of builds on Penny's Song of the Sirens approach but attempts to get around the idea that you could after midnight hire more strong men to undo whatever you set in motion before:

The key is in timing. My initial thought before reading Penny's suggestion was to try to construct some sort of mechanical device which you could attach to your mouth and had a timer set to force feed you the toxin exactly one minute after midnight. Certainly it would be possible to remove the device after midnight, but there must be some way of attaching it and securing it such that you could not do so within a minute, therefore even if you passed the test and were to attempt to foil the device, you would not have enough time to do so. I suppose you might be able to modify Penny's solution by suggesting the 10 strong men be present with you and instruct them to immediately feed you the toxin after passing the test, so you would not have time after midnight to find anyone willing to thwart those ten men.