Certain blood-thinning and cholesterol-lowering drugs may not be as great for you as they sound.

Regulators now say some patients may not metabolize the blood thinner Plavix well and cholesterol-lowering Zocor could lead to muscle damage.

The lack of a certain liver enzyme could prevent Bristol-Myers Squibb’s (Stock Quote: BMY) Plavix from converting to its active form in the body, preventing patients from seeing the full benefits of the drug, which is meant to prevent clots in blood vessels, according to the Food and Drug Administration.

The federal agency is now putting prominent warning labels on drug packaging to alert health professionals about the problem. Plavix is also sold under the generic name clopidogrel.

There is a test that can help determine whether a patient is a poor metabolizer of the drug, however.

Separately, the FDA says that high doses of Merck’s (Stock Quote: MRK) Zocor, Vytorin or Simcor could cause an increased risk of a muscle disease called myopathy, which could be a life-threatening side effect.

Studies have also found that daily aspirin use as a blood thinner to prevent heart attack and stroke should not be widely recommended, as MainStreet previously reported.

Cholesterol levels can be lowered naturally by exercising and eating more fruits, vegetables and fish, while garlic and ginger have been shown to help thin the blood and prevent heart attack and stroke, according to the National Institutes of Health.