A drug is defined as:
A substance recognized by an official pharmacopoeia or formulary.
A substance intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease.
A substance (other than food) intended to affect the structure or any function of the body.
A substance intended for use as a component of a medicine but not a device or a component, part or accessory of a device.
Biological products are included within this definition and are generally covered by the same laws and regulations, but differences exist regarding their manufacturing processes (chemical process versus biological process.)
Drugs@FDA Glossary of Terms
So, to be considered a drug, it doesn't have to necessarily "diagnose, treat, cure or prevent a disease." Being overweight, erectile dysfunction and thin eyelashes aren't a disease, yet there are FDA-approved products classified as drugs and available by prescription-only for those things. "Tobacco dependency" also isn't a disease, but it may be something people seek out a drug/medication to help them with and because nicotine is "intended to affect the structure or any function of the body of man," it meets the statutory definition of "drug" under Section 201(g) of the FD&C Act (21 USC 321(g)).
Classification of Products as Drugs and Devices and Additional Product Classification Issues