Medication that can be sold over the counter is generally considered to be safe for the majority of people, as long as they follow the manufacturer recommendations. These will need to be read closely in drugs that are more advance than the every day tylenol and ibuprofen (which also need to be understood well before they are used!), because there are many medical conditions and other medications that can be affected by starting a new over the counter medication. For example, if you have a heart condition and have been recommended that you take a blood thinner known as Plavix, some research has found that this medication is less effective when it is taken with some of the common over the counter acid suppression drugs. Reading the bottle carefully would explain this to you, but you do need to make sure to read the instructions and do speak with your doctor
if you have concerns, questions, or issues after reading those directions. There are also times when medications could be more effective when prescribed by a doctor. Regardless, if you do choose to start a new over the counter medication as per the manufacturer recommendations, this should be discussed with your doctor at your next appointment.