What to know about H2 blockers

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Medically reviewed by Femi Aremu, PharmD

Written by Steven O’Brien

H2 blockers are sometimes called H2 receptor antagonists, or H2RAs. They reduce the amount of acid that the stomach produces. This can help treat many common health issues, including gastroesophageal reflux disease (GERD), gastric ulcers, and occasional heartburn.

In the 1980s, H2 blockers were the treatment of choice for many acid-related stomach conditions. Doctors may now recommend other treatments.

Keep reading to learn more about the types of H2 blockers, their various uses, and some possible side effects.

What are H2 blockers? 

A chemical called histamine stimulates cells in the stomach lining to make hydrochloric acid. Too much of this acid can cause GERD and other painful conditions.

H2 blockers bind to histamine receptors in the stomach, reducing the amount of acid that the lining secretes.

H2 blockers usually offer relief from symptoms of an overproduction of stomach acid within 60 minutes of taking the medication, and the effects can last 4–10 hours.

H2 blockers can treat or prevent a number of health problems, including:

  • heartburn
  • GERD
  • duodenal and gastric ulcers
  • upper gastrointestinal bleeding
  • gastric hypersecretory diseases, such as Zollinger-Ellison syndrome

Types of H2 blockers 

The Food and Drug Administration (FDA) have approved the use of the following four H2 blockers. Each is available in at least one generic or branded form:


Cimetidine (Tagamet) comes in prescription, over-the-counter, injectable, and intravenous forms. A doctor can advise about the most effective formulation.

It is important to follow directions for use carefully. The right dosage depends on a person’s age, whether they are taking any other medications, and the form of the drug.

Older adults should refrain from taking this medication or use it with extreme caution. It may worsen or cause periods of confusion.

People with kidney or liver disease should consult a doctor before using Tagamet. Some common side effects include:

  • headaches
  • drowsiness
  • joint or muscle pain
  • breast swelling and tenderness (in females as well as males)
  • dizziness
  • confusion in elderly people, people with dementia, and those who are critically ill

In some cases, people may also experience:

  • a rash
  • an allergic reaction
  • production of breastmilk
  • trouble urinating
  • pancreatitis
  • kidney infection

Ranitidine hydrochloride

Ranitidine hydrochloride (Zantac) also comes in several forms, including prescription and nonprescription oral doses and an injectable solution.

The right dosage depends on a person’s age, their symptoms, any ongoing medications, and their kidney and liver function. It is best to consult a healthcare provider before taking this drug.

Also, it is important to follow instructions for use closely.

Some common side effects include:

  • headache, nausea, and dizziness, which are more common with Zantac than other H2 blockers
  • stomach pain
  • coughing
  • infection
  • diarrhea

The occurrence of mild side effects is slightly higher with this drug than with similar medications.

Doctors do not know how common more serious side effects are, but some have reported:

  • severe allergic reactions
  • depression
  • confusion
  • pancreatitis
  • hemolytic anemia
  • jaundice


Nizatidine (Axid or Tazac) comes as an oral solution or capsule.

The right dosage depends on a person’s age, their symptoms, and the form of the drug. Consult a doctor before taking it.

Like other H2 blockers, this medication may not be suitable for people with disorders of the kidney or liver. A doctor or another healthcare provider can offer specific guidance.

Some common side effects include:

  • congestion, a runny nose, and similar symptoms, which are more common with Axid than with other H2 blockers
  • a rash
  • drowsiness
  • irritability
  • anxiety
  • vomiting
  • diarrhea
  • a fever

On rare occasions, a person may experience more serious side effects, including:

  • hepatitis
  • anemia
  • jaundice
  • chest pain
  • impotence
  • seizures
  • a rapid heart rate
  • severe allergic reactions


Famotidine (Fluxid) comes in prescription and nonprescription oral formulas and as an intravenous infusion.

A doctor or another healthcare provider can recommend the right dosage based on the person’s symptoms, age, kidney function, and overall health.

Side effects can include:

  • diarrhea
  • dizziness
  • nausea
  • a loss of appetite

Rarely, people experience more severe side effects, including:

  • conjunctivitis, or pinkeye
  • hepatitis
  • depression
  • confusion
  • seizures
  • severe allergic reactions
  • Stevens-Johnson syndrome, a potentially life threatening skin disorder

General side effects

All four drugs pass into breastmilk, and researchers have yet to determine whether H2 blockers are safe for pregnant or nursing women to take. A doctor can provide more specific guidance.

H2 blockers can interact with other drugs. For example, Tagamet may alter the effectiveness of some pain relief medications. It may also reduce the metabolism of warfarin, a blood thinner.

These drugs can also affect the body’s ability to absorb vitamin B-12, vitamin D, folate, and some other nutrients. In addition, people who take H2 blockers and magnesium supplements should do so at different times of day to avoid the risk of interactions.

In general, anyone considering taking an H2 blocker should consult a healthcare provider first. However, these drugs are typically safe — only about 3% of people who take them experience serious side effects.

Depending on the underlying issue, a doctor may recommend a more effective treatment. For example, antibiotics are more effective at treating ulcers that result from infection with <em>Helicobacter pylori</em> bacteria.

Also, people may find relief from GERD symptoms by changing their diets. Read more about which foods to eat and avoid here.

H2 blockers vs. proton pump inhibitors

Proton pump inhibitors block the gastric proton pumps in certain stomach cells. This significantly reduces the amount of acid that the stomach can make. Some popular brands of this type of drug include Prilosec and Prevacid.

Proton pump inhibitors can be highly effective, and they may be more effective at managing GERD than H2 blockers. However, as with H2 blockers, a small number of people who take proton pump inhibitors experience side effects such as dizziness and headaches.

At the same time, an emerging body of evidence suggests that proton pump inhibitors may present serious risks. A 2017 longitudinal study, for example, found that veterans who used these drugs were more likely to die prematurely. These medications may also increase the risk of some bone fractures.

It is important to weigh the risks and benefits of H2 blockers and proton pump inhibitors. Discuss the available options, including lifestyle adjustments, with a healthcare provider before starting a course of treatment.


GERD, ulcers, and other conditions related to stomach acid can be very painful. Various H2 blockers may help with symptoms, but each can cause side effects. Also, proton pump inhibitors may be more effective than H2 blockers at treating GERD.

It is important to consult a doctor before embarking on treatment. The doctor can correctly identify the underlying issue and recommend the best course of action after taking the person’s medical history into account.

While most H2 blockers only manage symptoms, some, such as Axid, can also treat duodenal ulcers, thus resolving symptoms. Dietary and lifestyle adjustments may also help.

Kayode Ojo
Kayode Emmanuel Ojo is the Co-Founder and Managing Director at SHEFFA Limited. He is presently studying Computer Science at the National Open University, Victoria Island, Lagos.