According to Wikipedia...
Disadvantages of Invisalign
Despite the considerable cosmetic advantage of Invisalign, the product also has many disadvantages: the very fact that the aligners are removable means they are not continually correcting the teeth. Unlike traditional fixed braces, they are largely dependent on a patient's habits and their consistency in wearing the aligners.
The aligners must be removed for eating. They and the teeth should be cleaned before re-inserting afterwards. This can be troublesome for people who are constantly snacking and may mean the patient has to clean their teeth up to six times daily. However, this tends to discourage snacking, which some people might find useful.
Unlike traditional braces, if a patient grinds or clenches his or her teeth during the day or while sleeping, the aligners or braces can become damaged. In practice, however, this problem is very rare and a new aligner can be ordered. Also, similar to traditional appliances, aligners may cause a slight lisp at the beginning of treatment. This usually disappears as the patient becomes used to the treatment.
The aligners are constructed of implantable grade plastic, however a growing number of reported cases of allergic and toxic sensitivity reactions to Invisalign are being reported to the FDA and online information forums like Archwired.[citation needed] Minor symptoms such as sore throat, cough and nausea have been reported.[citation needed] In more serious cases, the FDA has received reports of systemic swelling or throat pain that has extended to the upper chest and wind passages requiring emergency medical treatment and discontinuation of the Invisalign treatment. It is more than likely that this is all a case of people blaming their illnesses on whatever lifestyle changes they have made recently. None of the medical issues above have been upheld by the authorities.[citation needed] While the Invisalign company provides no information except the MSDS (material safety data sheet) directly to patients or orthodontists, working through the patient's orthodontist Invisalign will make the aligners with several different materials to attempt to reduce toxic or allergic sensitivity.