PAC stands for proanthocyanidins, a group of natural compounds found in high concentrations in cranberries. They’re the active ingredient responsible for cranberry’s well-known benefits for urinary tract health. When you see “36 mg PAC” on a supplement label, it refers to the amount of these compounds per dose, which is the threshold most commonly used in clinical research for preventing urinary tract infections.
What PACs Actually Are
Proanthocyanidins are a type of plant compound built from smaller molecules called flavan-3-ols, which link together in chains. What makes cranberry PACs special is their specific structure: they’re classified as A-type proanthocyanidins. Most other PAC-rich foods, like grapes, apples, and chocolate, contain B-type PACs, which have a slightly different chemical bond between their building blocks. A-type PACs have an additional connection point between their molecular units, and this structural difference directly affects how they behave in your body.
Very few foods contain A-type PACs in meaningful amounts. Plums, peanuts, avocados, cinnamon, and lingonberries have some, but cranberries remain the richest dietary source by a wide margin. This is why cranberry supplements specifically highlight PAC content on their labels: it’s the compound that sets cranberry apart from other berry or fruit extracts.
How PACs Work Against UTIs
The bacteria behind most urinary tract infections, E. coli, use tiny hair-like projections called P-fimbriae to latch onto the walls of the bladder and urinary tract. Once attached, the bacteria multiply and trigger infection. A-type PACs from cranberries interfere with this process by blocking the bacteria’s ability to grip onto those cell walls. Without a foothold, the bacteria get flushed out during normal urination instead of building up into an infection.
This is not the same as killing bacteria. PACs don’t work like an antibiotic. Instead, they act as a kind of non-stick coating for your urinary tract lining. Research on cultured bladder cells has confirmed that cranberry PAC extract inhibits E. coli adhesion in a dose-dependent way, meaning more PAC produces a stronger anti-adhesion effect.
The 36 mg Dosage Threshold
The number you’ll see most often on cranberry supplement labels is 36 mg of PAC per day. This figure comes from clinical trials showing that 36 mg of cranberry PAC equivalents per day is the minimum effective dose for producing a measurable bacterial anti-adhesion effect in urine. Some research suggests that 72 mg per day may offer better protection in certain cases, but 36 mg remains the standard benchmark.
Timing matters too. Researchers have recommended splitting the daily dose into two servings, one in the morning and one at night. A clinical trial tracking anti-adhesion activity in urine found that the protective effect peaked between 3 and 6 hours after taking a cranberry dose, then gradually faded. By 24 hours after a single dose, the effect had returned to near baseline. Taking a second dose about 12 hours after the first extended meaningful protection for a full 24-hour cycle. So a single daily capsule may leave gaps in coverage that twice-daily dosing avoids.
Juice vs. Extract Supplements
A standard recommendation for UTI prevention has been drinking 300 mL (about 10 ounces) of cranberry juice cocktail daily, which delivers roughly 36 mg of PAC. Concentrated supplements can pack the same amount or more into a small capsule, which is why many people prefer them.
Not all supplements deliver the same quality of PAC, though. The active compounds need to be soluble to work. A study comparing two types of commercial cranberry supplements found that products made from cranberry juice extract had higher soluble PAC content than those made from whole cranberry blends, which contained mostly insoluble PACs. Only the soluble fraction showed anti-adhesion activity in testing. If a supplement is made from whole cranberry powder rather than a juice-derived extract, it may contain PACs that your body can’t effectively use.
How PAC Content Is Measured
When a supplement label claims a specific PAC amount, that number typically comes from a testing method called BL-DMAC, which is the official reference standard for cranberry products. This method reacts with the relevant compounds and measures their concentration. It’s the basis for the 36 mg daily recommendation used in clinical trials.
However, BL-DMAC has known limitations. It can’t distinguish between cranberry’s beneficial A-type PACs and the common B-type PACs found in grapes or cocoa. A product could theoretically contain B-type PACs from a non-cranberry source and still register a PAC value on this test. The method also tends to underestimate actual PAC content in more complex extracts. Some quality-testing labs now use additional techniques that can identify the specific ratio of A-type to B-type PACs as a better indicator of genuine cranberry content. For consumers, this means PAC numbers on labels aren’t always directly comparable between brands.
What the Clinical Evidence Shows
A large Cochrane review, widely considered the gold standard for medical evidence, analyzed 50 randomized trials involving nearly 9,000 participants. Cranberry products reduced the overall risk of symptomatic, lab-confirmed UTIs by about 30%. The benefits were strongest in three groups: women with recurrent UTIs saw a 26% risk reduction, children experienced a 54% reduction, and people susceptible to UTIs after medical procedures like bladder radiotherapy had a 53% reduction.
The picture wasn’t uniformly positive across all populations. Elderly people in institutional care showed little to no benefit. The same was true for pregnant women and adults with neurological conditions affecting bladder emptying. The evidence currently supports cranberry PAC supplementation most strongly for otherwise healthy women who get frequent UTIs and for children prone to infections.
Safety and Drug Interactions
Cranberry PAC supplements are generally well tolerated, but one interaction worth knowing about involves the blood thinner warfarin. The UK’s Committee on Safety of Medicines has flagged that large volumes of cranberry juice can destabilize warfarin therapy, potentially making the drug’s blood-thinning effect unpredictable. Small amounts of juice or a standard supplement dose are not expected to cause problems, but if you take warfarin, it’s worth mentioning cranberry use to your prescriber so they can monitor your levels if needed.