The use of over the counter medications for uses other than those recommended can have very serious consequences. While some medications may not have a negative effect on all people, some people can experience them whether it's an issue of dosagae or of medication type.
Some medications contain ingredients that are harmful to people with certain conditions and others can cause problems if taken in quantitites greater than the recommended dosage. For example, acetaminophen (brand name Tylenol) is safe under normal dosages but excessive dosages can cause liver problem even in those with normal liver functions.
Other medications, such as cold/cough medicines, can also have serious consequences. Check the links below for more details but they include high blood pressure, hallucinations, seizures, and brain damage. Taking high dosages of cold and cough medicines is even riskier because many of them containe multiple medications further complicating the consequences and effects on the body.
In general, drugs are not sold without prescriptions if they have the potential to be addictive or if they are liable to abuse. The main dangers from most OTC drugs are of overdoses or of bad drug combinations. If users do not follow the directions accompanying the drugs concerning maximum safe doses, they can experience serious side-effects or even death. For example, someone might take more than the recommended dose of an NSAID such as ibuprofen for pain not realizing the potential for stomach damage.
Many drugs are safe when taken on their own but not when combined with alcohol or other similar drugs. For example, taking antihistamine or muscle relaxants and then drinking alcohol may make it unsafe to drive even though you would able to drive while taking either separately.
Finally, adult doses of OTC drugs (or vitamins) should never be left in reach of children, as the normal adult dose can kill or severely injure a small child.