Almost all of us need prescription medicine at one time or another — and some of us, especially older folks, rely on a whole pharmacy of pills to keep us healthy. Along with drugs come side effects. While most side effects caused by medications are mild (drowsiness and upset stomach are among the most common), more serious drug reactions send some 1.5 million Americans to doctors’ offices and hospital emergency rooms each year, according to the American Medical Association.

A Harvard University study found that five groups of medication trigger the most problems. Read on to find out which ones they are.

Should you stop taking your prescription med? Of course not. Your doctor wouldn’t have prescribed it if she didn’t think the benefits outweighed the risks. (If you have doubts, discuss the potential dangers together.) But do be aware of the possible side effects, and know the signs of trouble to watch for.

Your best defense against a hazardous drug reaction? Information. That means discussing potential side effects of a drug with your doctor, nurse practitioner and pharmacist before you start taking it.

Related: Foods to Avoid When You're on Medication

If you experience a serious allergic reaction, call 9-1-1 or go to an emergency room. The American Academy of Allergy, Asthma & Immunology provides a list of serious reaction, which include hives, face or throat swelling, trouble breathing, chest tightening and passing out.

Here are the five troublesome drug types and what you should know, and do, if you’re taking a medication in any of these groups.

1. Antibiotics

Reactions to drugs such as penicillin, sulfa drugs and erythromycin send more than 140,000 people to emergency rooms and doctor’s offices each year, according to the Centers for Disease Control and Prevention (CDC). One in three people who take antibiotics may develop diarrhea. (Eating yogurt or taking a probiotic supplement recommended by your doctor may help prevent it).

Tell your doctor if you develop nausea, vomiting or thrush, an overgrowth of yeast that can leave white patches on your tongue.

Get immediate help if you experience signs of a serious allergic reaction.

Related: High Blood Pressure? Read This Before Taking Cold Meds

2. Hormones

A 2014 CDC study found that just one commonly prescribed hormone — insulin, prescribed for diabetes — causes more than 97,000 emergency room visits a year, most of them for severe low blood sugar problems such as shock, seizures or loss of consciousness. Other hormones that can cause problems include steroids used for asthma, allergies and autoimmune diseases such as multiple sclerosis and lupus. Women should be mindful about estrogen and progesterone, commonly prescribed to ease menopause symptoms. And men should beware the risks of testosterone for "low T." 

Tell your doctor if you experience headaches, dizziness, sleep changes, nausea or vomiting.

Get immediate help for vision changes, severe abdominal pain or headache, seizures, loss of consciousness, leg pain, swelling, unusual bleeding or fast heartbeat.

3. Heart drugs

According to the Food and Drug Administration’s adverse drug event reporting system, medications that control blood pressure drugs called beta blockers rank in the top 10 for drugs with the most reported side effects. Cholesterol-lowering drugs, such as statins, may also cause reactions.

Related: Cholesterol-Lowering Statins: What to Know Before You Fill Your Prescription

Tell your doctor if you have sleep problems, tiredness, erectile dysfunction, cold hands and feet, cough, dizziness, headache, palpitations and or a drop in blood pressure when you stand up.

Get immediate help for shortness of breath, wheezing, fainting, unusual heartbeat, vision changes, joint pain or swelling, and signs of severe dehydration (not urinating or very dark yellow or amber-colored urine, dizziness or lightheadedness, unconsciousness or delirium).

Tell your doctor if you experience constipation, stomach pain, nausea, headache, memory loss or confusion.

Get immediate help for muscle pain or weakness, dark or red urine, yellowing of the skin or eyes, joint pain, fever or chills or loss of appetite.

4. Mood and anxiety drugs

Antidepressants, anti-anxiety drugs and medication to control convulsions (which are also used to treat mood disorders) send nearly 90,000 people to U.S. emergency rooms annually, according to a 2014 CDC study.

Tell your doctor if you have appetite changes, nervousness, nausea, anxiety, drowsiness, unwanted mood changes, dry mouth, weight changes or sweating.

Get immediate help for fever, joint pain, signs of an allergic reaction, seizures, hallucinations, a fast or irregular heartbeat and confusion.

5. Pain medicines

Prescription pain drugs including codeine, oxycodone and morphine send more than 400,000 people to the emergency room each year, according to a 2014 drug- reactions report from the U.S. Department of Health and Human Services. But even aspirin or ibuprofen can cause a bad reaction in some cases.

Tell your doctor if you have constipation, nausea, drowsiness, headache or difficulty urinating.

Get immediate help for signs of addiction, digestive-system bleeding (black or bloody stools, vomit that contains blood or that looks like coffee grounds), loss of hearing, breathing problems, a fast or irregular heartbeat, confusion, sleepiness, cold and clammy skin, dizziness, or fainting or loss of consciousness.

Related: Think Twice Before You Take a Prescription Painkiller

Remember, serious side effects are rare. But being able to recognize them and get treatment is your best protection.

Sari Harrar is an award-winning health, medicine and science journalist whose work appears in Dr. Oz The Good Life magazine, Good Housekeeping, O--Oprah Magazine, Organic Gardening and other publications.