How much garlic do you need per day?
Research shows that the daily dose of garlic depends on what you want to achieve. For raised blood pressure the evidence is strongest: more than 1200 mg of aged garlic extract per day, for at least 8 to 12 weeks.
Garlic is being studied for two very different purposes, and the amount you need differs depending on what you are trying to achieve. For improving blood fats, blood pressure or inflammation markers, supplements are what the research looks at, not the cloves in your food.
For cholesterol and blood fats: 600 to 900 mg of dried garlic powder per day lowered total cholesterol by an average of 12 percent compared with a dummy pill in 16 studies combined, with effects visible after one month and lasting for at least six months. However, a later analysis combining 13 other studies found the exact opposite: no measurable effect on cholesterol, LDL, HDL or blood fats. The two analyses therefore contradict each other, making a firm dose recommendation for cholesterol impossible. An effect is most likely to be seen in people who already have mildly raised levels (total cholesterol above 200 mg/dL). If you want to try it, use it for at least 8 to 12 weeks and discuss it with your doctor.
For lowering raised blood pressure, higher doses have been studied: more than 1200 mg per day of aged garlic extract, or an optimised extract containing the active compound S-allylcysteine. In practice, this is clearly the best-supported use. Use it for at least 8 to 12 weeks. At a normal blood pressure, garlic has no demonstrable effect. If your blood pressure is raised, always discuss this with your doctor.
A third use that has been studied: allicin, the active compound in garlic, as an addition to the standard treatment for the stomach bacterium Helicobacter pylori. Eight studies combined showed a higher success rate for eliminating the bacterium (93 percent vs. 84 percent) and better healing of stomach ulcers. The researchers themselves rated the quality of this evidence as 'low' to 'very low'. This is definitely something to discuss with your doctor, not something to start on your own.
For other popular claims, such as boosting the immune system or improving the gut microbiome, no reliable dose recommendation exists. The evidence for the microbiome comes almost entirely from animal studies and has not yet been confirmed in people.
This answer is based on three analyses that combine the results of multiple studies. The cholesterol findings are contradictory: one analysis of 16 studies found a clear effect, while another of 13 studies did not. The researchers behind the second analysis pointed to too much variation between studies for a clear conclusion to be drawn. For the stomach bacterium application, the researchers themselves rated 41 of the analyses as low or very low quality, so the evidence there is thin. For blood pressure, the best-supported figures come from the reference card rather than from the three abstracts provided.