The short answer to the question “are occasional drivers required to drug test” is Yes.

All drivers that operate a commercial motor vehicle, as defined in 49 CFR §382.107, which requires a driver holding a commercial driver’s license, are subject to the Drug and Alcohol testing requirements in 49 CFR Parts 40 and 382. (See 49 CFR §383.3). This includes, but is not limited to: full time, regularly-employed drivers; casual, intermittent or occasional drivers; leased drivers and independent owner-operator contractors. See federal register notice on leased drivers.

https://www.fmcsa.dot.gov/regulations/drug-alcohol-testing/occasional-drivers-my-company-purchased-large-truck-over-26001-lbs

See this:

Occasional Drivers: My company purchased a large truck that is over 26,001 lbs. gross vehicle weight rating (GVWR). We do not currently employ any commercial driver’s license (CDL) drivers. The vehicle has only been driven a few times by a family friend, who is not an employee and has driven the vehicle as a favor absent any compensation. Does our company need a DOT drug and alcohol testing program to test this occasional driver?

Yep.

In accordance with §382.103, your company must implement a DOT drug and alcohol program for all drivers operating a commercial motor vehicle (CMV) that requires the driver to possess a commercial driver’s license (CDL).  Section §382.107 defines “Driver” as “any person who operates a commercial motor vehicle.”  This includes, but is not limited to: full time, regularly-employed drivers; casual, intermittent or occasional drivers; leased drivers and independent owner-operator contractors.