According to the Merck Manual, an adverse drug reaction (ADR) is "any drug effect that is unwanted, unpleasant, noxious or potentially harmful." Considering that most drugs have more than one effect coupled with the fact that more than 3.5 billion prescriptions were filled in the U.S. in 2008, it is not surprising that ADRs are common. The list of potential ADRs is extensive and ranges in severity from mild to life-threatening. Fortunately, most common ADRs are mild.
Stomach Upset
Medicines used for a variety of disorders can cause stomach upset. Possible symptoms include queasiness, nausea, vomiting, belching, heartburn and loss of appetite. Anti-inflammatory and pain medicines are common classes of drugs known to cause possible stomach upset.
Intestinal Upset
Medications can affect your usual bowel habits by causing diarrhea or constipation. Antibiotics and chemotherapy drugs commonly cause diarrhea. Narcotic painkillers and medicines that decrease muscle activity in the intestines may cause constipation.
Fatigue
Fatigue--extreme tiredness that is not explained by lack of sleep or overexertion--is another common ADR. Blood pressure medicines and immunoregulatory drugs are two classes of drugs that may cause fatigue as an unwanted side effect.
Drowsiness
Whereas fatigue is a feeling of utter exhaustion, drowsiness is intense sleepiness that can make it difficult to stay awake. Drowsiness is a common ADR, and one to be particularly careful of when starting a new medication. Examples of types of drugs that can cause drowsiness include antihistamines, opioid painkillers and antiseizure medicines.
Headache
Headache is a common ADR, which can be experienced with a variety of medications including drugs to treat attention deficit hyperactivity disorder (ADHD) and certain heart medicines.
Jitteriness, Nervousness and Insomnia
Some nasal decongestants and certain asthma medications can cause jitteriness, nervousness and insomnia.
Increased Urinary Frequency
Diuretics are commonly used alone or in combination with other drugs to treat high blood pressure and heart problems. This class of drugs causes increased urinary frequency, which can be quite bothersome.
Drug Rash
An estimated 5 percent to 10 percent of ADRs are true drug allergies. A skin rash, called a drug rash, is the most common form of allergic skin reaction to medication. It typically appears on the trunk within one to three weeks after initial exposure to the offending drug, and often spreads to the arms and legs.
Dry Mouth
Medicines used to treat gastrointestinal spasms, such as those experienced by people with irritable bowel syndrome, and motion sickness medications generally cause dry mouth. Like most ADRs, this side effect resolves when the medication is stopped.
Altered Sense of Taste
Many classes of drugs can alter the sense of taste, with unpleasant results. Common types of agents that may cause this ADR include medications to treat an overactive thyroid, certain antibiotics and drugs used to treat rheumatoid arthritis.
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