It isn't illegal to chew gum, but it is illegal to bring them into Singapore or sell them except for medical purposes. Since you can't buy gum (selling is illegal) there's no way to chew them so it's almost as good as not being able to chew gum. See this: http://www.customs.gov.sg/leftNa...
Definition: In Vitro. A study done in vitro, in contrast, looks at how a drug or procedure responds in the confines of a test tube or laboratory dish. Clinical trials or medical studies may be performed either in vivo or in vitro.
Scope This document applies to all medical devices, including in vitro diagnostic medical devices. 1.4. Definitions Definitions that do not indicate they are set out in the Health Products Act 2007 (Act) or Health Products (Medical Devices) Regulations (Regulations) are intended as guidance in this document.