This is a thyroid forum, so I wonder if your question is in relation to a thyroid condition?
Although there may be some OTC med or illegal drugs that may skew certain blood tests because they affect your body in different ways or are processed differently, doctors would, most likely, need to do a specific test (blood, urine or breath) to know what you might be taking. For instance, if you're taking a marijuana product, they'd have to test for THC in your blood to know that, likewise, for other drugs.
Taking OTC supplements, such as iron, Vitamin B-12, etc could mask certain other issues or make it appear as though one has problems that don't exist, such as too much iron appearing to be the condition called hemochromatosis, higher than normal doses of ibuprophen may lower blood glucose levels, etc.
The bottom line is that for your own safety, it's always test to be upfront with your doctor regarding the medications (OTC or otherwise) you're taking since some medications may increase/decrease blood glucose, some may affect liver function, others may affect kidney function, etc, depending on how the body processes the substance. You don't do yourself any favors by hiding things from your doctor because skewed results may look like there's something wrong that isn't or may miss something that is wrong.