What Is THC? Potency Labels, THC vs CBD and How Much to Start With

THC is the reason cannabis feels the way it does. It is also the number most shoppers look at first on a menu, and the one that is easiest to misread. This guide explains what THC is, how to read THC on a label, how it differs from CBD and THCA, and how timing changes between smoking, vaping and edibles.

What THC is

THC, short for delta-9-tetrahydrocannabinol, is one of more than 100 cannabinoids found in the cannabis plant. It is the main intoxicating one: it is what produces the “high”. Most of it sits in the sticky resin glands, called trichomes, that cover the buds.

Fresh cannabis contains very little THC. It mostly contains THCA, an acid form that does not get you high. Heat turns THCA into THC, a step called decarboxylation. That is why smoking, vaping and baking work, and why eating raw flower does very little.

How to read THC on a label

Product	How THC is shown	Example
Flower and pre-rolls	Percentage, or mg per gram (10% = 100 mg/g)	“THC 22%” means about 220 mg of THC per gram
Edibles and drinks	Milligrams per piece and per package	“10 pieces × 10 mg” = 100 mg in the pack
Vapes and concentrates	Percentage of the oil or extract	Distillate is often 80–90% THC
Oils and tinctures	mg per mL	“30 mg/mL” means 30 mg in a full dropper of 1 mL

Two tips make labels easier. First, for edibles, look at THC per piece, not the big number on the front of the pack. Licensed edibles in Canada are capped at 10 mg of THC per package, but many products sold online list totals in the hundreds or thousands of milligrams, so one piece can be far stronger than a newcomer expects. Second, a higher percentage on flower does not automatically mean a better experience. Smell, freshness and terpenes matter too, which is why grades like AA to AAAA look at more than THC.

THC vs CBD vs THCA

	THC	CBD	THCA
Gets you high?	Yes	No	No, until heated
Where you find it	Heated flower, vapes, edibles, extracts	Hemp and cannabis products, often paired with THC	Raw, unheated flower
On the label	% or mg	% or mg	Often listed as “total THC potential”

CBD is the other well-known cannabinoid. It does not cause a high, and it is sold on its own or mixed with THC, for example in 1:1 gummies. Health Canada suggests that people who want a milder experience choose products with a low amount of THC and an equal or higher amount of CBD.

How fast THC kicks in, by method

Method	Starts	Full effect	How long it lasts
Smoking or vaping	Seconds to minutes	Up to 30 minutes	Usually a few hours
Edibles and drinks	30 minutes to 2 hours	Up to 4 hours	Up to 12 hours, with lingering effects up to 24 hours

The edible timeline is the one that catches people out. Because it can take two hours to feel anything, it is easy to take a second piece too early. Our guide on how long edibles take to kick in goes into more detail.

How much THC to start with

Health Canada’s lower-risk advice is simple: start low and go slow. For edibles, it suggests products with 2.5 mg of THC or less, then waiting to feel the effects before having more. For smoking or vaping, it suggests starting with 1 or 2 puffs of flower or a vape with 10% THC or less. Everyone reacts differently, so treat these as starting points, not targets.

• Use a microdose (around 2.5 mg) the first time you try a new edible.
• Cut strong gummies into halves or quarters. A 100 mg piece cut in quarters is still 25 mg per quarter.
• Avoid concentrates like shatter and hash until you know how you react. Health Canada notes they contain a high concentration of THC.
• Do not mix cannabis with alcohol, and never drive after using it.

Is THC legal in Ontario?

Adults 19 and older can buy, possess and use recreational cannabis in Ontario, within federal and provincial limits, such as 30 grams of dried cannabis in public. Where you can smoke it has its own rules. See where you can smoke weed in London, Ontario.

Frequently asked questions

What does THC stand for?

THC stands for tetrahydrocannabinol, more precisely delta-9-tetrahydrocannabinol. It is the main compound in cannabis that causes a high.

What does 20% THC mean?

It means about 20% of the dried flower’s weight is THC, or roughly 200 mg of THC in every gram.

How much THC should a beginner take?

Health Canada suggests edibles with 2.5 mg of THC or less, or 1–2 puffs of flower or a vape with 10% THC or less, and waiting to feel the effects before having more.

Is THCA the same as THC?

No. THCA is the raw acid form found in fresh cannabis and does not get you high. Heat from smoking, vaping or baking turns it into THC.

How long does THC from an edible last?

Effects can start in 30 minutes to 2 hours, peak around 4 hours and last up to 12 hours, with lingering effects up to 24 hours.

Want to compare products? Browse flower, edibles and disposable vapes, or look up any term in our cannabis glossary. Cannabis is for adults 19+ in Ontario.

Buy weed online in Canada · Same-day weed delivery in London, Ontario on weekday orders placed before noon (12:00 PM EST).