One of the problems with seeking a therapist when you are multiple is that there's a good chance that you're going to be misdiagnosed -- or already have been. If you know that you're multiple already and seek out treatment, then you might be luckier than some, but that doesn't guarantee that the therapists will understand or believe you. A good article giving an example case of misdiagnosis is at http://www.fortea.us/english/psiquiatria/alternative.htm and deserves reading by therapeutic staff.
Dangers of misdiagnosis include prescribing medications to multiples which may be entirely inappropriate to them, and many psychopharmaceuticals are dangerous or have strong side effects, especially for multiples who often can't get consistent results across all their residents when they take an analgesic (headache medicine). For more information on multiples and medications, try this article.
At the same time it is plausible for a multiple to have two or more psychiatric issues (comorbidity), in which case most or all alters will have the diagnosis in question. One good example of this is depression or generalized anxiety disorder. In cases where a mood disorder seems pandemic -- applies across almost everyone -- it is usually responsive to medicine. All medication for multiples should be viewed with caution and stopped if there are adverse effects to the system.
There are no known psychoactive medications for multiples, although a few of them may help calm down rapid switching, which generally is caused by anxiety (which is a physiological reaction) anyhow.