The British Medical Association (BMA) warned some of the tests were unregulated and put extra pressure on GPs left to reassure their patients.
It comes after a BMJ report found some companies were making misleading claims about what their tests could identify.
The BMJ said it referred two firms to the UK Advertising Standards Authority.
People can pay private companies for a range of tests that can be done at home or at a private clinic, ranging from cholesterol and blood sugar level checks to tests looking for genetic abnormalities or signs of cancer.
Some private companies claim their tests can help to reduce the burden on the NHS and provide clinical advice alongside the results.
Other companies suggest people should go to their own doctor if the results are abnormal.
Dr Preeti Shukla, BMA GP committee clinical and prescribing lead, said family doctors and their colleagues were already facing unsustainable pressure trying to meet the needs of their most vulnerable patients.
She said: "The rise of private companies offering over-the-counter, unregulated blood tests that make dubious claims about what they can identify, is a real concern for GPs and their NHS colleagues, who are the ones who are often left to interpret results and reassure their patients."
Providing people with test results with no context, explanation or follow-up arrangements can cause "unnecessary worry and anxiety", Dr Shukla added.
She explained: "It puts GPs in an incredibly difficult position if they are asked to interpret and explain results of tests that they have not initiated, and make decisions based on them."
The BMA recommends anyone with worrying symptoms or health concerns contact their GP practice, as well as make use of regulated testing and screening programmes available through the NHS.
A spokesperson for NHS England said: "At a time when GP teams are delivering tens of millions of appointments every month, additional pressure should not be put on their workload from potentially misleading information.
"People should use trusted sources of information, such as the NHS website, and when feeling unwell, coming forward to contact the NHS for expert advice as they usually would."