Just as anyone should when considering using ANY drug, OTC or otherwise.
For example, Immodium is popular
for detox
for the inevitable bathroom issues. However, it should be used very cautiously with people who have a history of liver disease, those who have diarrhea caused by antibiotics, and those who are taking certain anti-virals. Another example is the use of herbal supplements. They are contraindicated with countless prescripition medicines.