A plague of spam is occuring right now via thousands of hacked Twitter accounts which advertises a weight-loss product. It is not known how all these accounts have been hacked, but given the mass that's been affected and the amount of spammy msgs, it would seem the users' passwords have been stolen.
Below is what it looks like :-
...with the link pointing to a website that advertises an Acai Berry dietary supplement.
Needless to say, if you've spotted the above message coming from your own account, you should immediately change your password.
Another way to help prevent this kind of thing happening to you in future is to log in to your Twitter account, click on "Settings" in the upper-right dropdown menu, then choose "Connections" or "Applications" and "Revoke Access" which will revoke Twitter access to any suspicious apps thereon.