How is this legal?

Delta-9 THC & THCa

The Farm Bill

The 2018 Farm Bill, officially known as the Agriculture Improvement Act of 2018, removed hemp and its derivatives from the Controlled Substances Act, thereby making hemp a legal agricultural product. 

This legislation allows the sale of products containing less than 0.3% Delta-9 THC by dry weight. Importantly, the regulation applies only to Delta-9 THC, rather than total THC. Cannabis naturally produces significantly more THCa than Delta-9 THC, with some strains containing up to 25% to 30% THCa, while only having about 0.2% Delta-9 THC.

THC

Tetrahydrocannabinol (THC) is the main psychoactive component of cannabis. It is known for creating the euphoric "high" sensation associated with cannabis. Delta-9 THC interacts with the body's endocannabinoid system, leading to these effects.

The two primary forms of naturally occurring THC are Delta-9 THC and THCa, while other variants such as Delta-8-THC and THCP occur in trace amounts and are typically produced in laboratories.

THCa

In its raw state, THCa is non-psychoactive. However, it degrades over time or when heated—such as when it is smoked or vaporized—undergoing decarboxylation and converting into the psychoactive Delta-9 THC.
Essentially, all cannabis is primarily THCa, with only minimal amounts of already-psychoactive Delta-9 THC present. This is why we smoke cannabis or need to decarboxylate it to activate the psychoactive compounds!

Delta-9 THC

Delta-9 THC (∆-tetrahydrocannabinol) is the active psychoactive compound in the cannabis plant, found in small quantities. It is responsible for promoting relaxation, elevating mood, and enhancing sensory perception.
Growers are required to test their plants 30 days before harvest, which allows skilled cultivators to maintain Delta-9 THC levels within the legal limit of 0.3%by total weight.
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Sources

H.R.2 - 115th Congress (2017-2018): Agriculture Improvement Act of 2018, H.R.2, 115th Cong. (2018), https://www.congress.gov/bill/115th-congress/house-bill/2.
Mayo Clinic Staff 2025 Mayo Foundation for Medical Education and Research (MFMER). Marijuana. https://www.mayoclinic.org/drugs-supplements-marijuana/art-20364974
2024 Rice University’s Baker Institute for Public Policy: Breaking Down Popular Cannabis Compounds. https://doi.org/10.25613/D0GY-HZ81 
Ng T, Gupta V, Keshock MC. Tetrahydrocannabinol (THC) [Updated 2023 Nov 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan. https://www.ncbi.nlm.nih.gov/books/NBK563174/
Cannabis (marijuana) and cannabinoids: What you need to know. National Center for Complementary and Integrative Health. https://www.nccih.nih.gov/health/cannabis-marijuana-and-cannabinoids-what-you-need-to-know.