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Overview: Potassium acid phosphate is used for certain types of kidney stones, among other uses. It works by increasing the amount of acid in urine, which makes it harder for kidney stones to form. The most common side effects are nausea, vomiting, diarrhea, and stomach pain.

Uses

What is potassium acid phosphate used for?

Potassium acid phosphate is commonly used for the following conditions.

  • Certain types of kidney stones
  • Reduce the smell and irritation caused by high levels of ammonia in urine (pee)
  • Increase the effects of a medicine called methenamine, which is used to treat kidney and bladder infections

Potassium acid phosphate may also be used for other conditions as determined by your health care provider.

Note: The FDA has not reviewed this medicine for safety and effectiveness, and the label is not approved by the FDA. Learn more about unapproved drugs here.

How does potassium acid phosphate work (mechanism of action)?

Potassium acid phosphate helps to increase the amount of acid in your urine (pee). This makes it harder for kidney stones to form or for bacteria to survive. It also helps to reduce the smell and irritation caused by ammonia in urine.

How is potassium acid phosphate supplied (dosage forms)?

Brands and other names

  • K-Phos Original

Dosage forms and strengths

  • 500 mg oral tablets

How should I store potassium acid phosphate?

Potassium acid phosphate should be stored at room temperature, between 68 F and 77 F (20 C and 25 C). Store in a cool, dry place and keep the container tightly closed.

Side Effects

What are the most common side effects of potassium acid phosphate?

The most common side effects of potassium acid phosphate are listed below. Tell your health care provider if you have any of these side effects that bother you.

  • Nausea or vomiting
  • Diarrhea
  • Stomach pain

There may be other side effects of potassium acid phosphate that are not listed here. Contact your health care provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088 (800-332-1088). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of potassium acid phosphate?

While less common, the most serious side effects of potassium acid phosphate are described below, along with what to do if they happen.

Severe Allergic Reactions. Potassium acid phosphate may cause allergic reactions, which can be serious. Stop taking potassium acid phosphate and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Electrolyte Changes. Potassium acid phosphate may cause changes to the levels of certain minerals in your body called electrolytes. For example, it may cause high potassium levels (hyperkalemia), high phosphorus levels (hyperphosphatemia), or low calcium levels (hypocalcemia). These changes can be serious. The risk is higher in people who take certain other medicines or who have certain health problems. Keep appointments for blood tests so that your health care provider can check your electrolyte levels. Call your health care provider if you have any of the following symptoms of electrolyte changes.

  • Muscle cramps, spasms, or unwanted movements
  • Muscle weakness or unusual tiredness
  • Bone and joint pain
  • Rash or itching
  • Numbness or tingling around the mouth
  • Slow heartbeat or a feeling of skipped heartbeats
  • Nausea or vomiting
  • Trouble breathing
  • Abnormal heart rhythm

Warnings & Precautions

Who should not use potassium acid phosphate?

Allergies to Ingredients. People who are allergic to any of the following should not take potassium acid phosphate.

  • K-Phos Original
  • Potassium acid phosphate 
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific potassium acid phosphate products they stock.

High Phosphorus Levels.You should not take potassium acid phosphate if you have high phosphorus levels.

High Potassium Levels. You should not take potassium acid phosphate if you have high potassium levels.

Serious Kidney Problems. If you have serious kidney problems, you should not take potassium acid phosphate.

Certain Kidney or Bladder Infections. Certain types of infections can cause phosphate stones to form in the kidneys. If you have phosphate stones caused by one of these infections, you should not take potassium acid phosphate.

What should I know about potassium acid phosphate before using it?

Do not take potassium acid phosphate unless it has been prescribed to you by a health care provider. Take it as prescribed.

Do not share potassium acid phosphate with other people, even if they have the same condition as you. It may harm them.

Keep potassium acid phosphate out of the reach of children.

Each potassium acid phosphate tablet should be dissolved in about one cup (240 mL) of water. Stirring the mixture will help the tablets to dissolve faster, although it may take more than five minutes for the tablets to dissolve completely. If any pieces of the tablet do not dissolve, they may be crushed and then stirred in. It is best to drink your dose of potassium acid phosphate with a meal or snack.

If you are taking potassium acid phosphate for kidney stones, you may still pass old stones that were already present before you started this medicine. This medicine works by helping to prevent new stones from forming.

Your health care provider may monitor your blood levels while you are taking potassium acid phosphate. Keep all appointments for blood tests and other tests.

What should I tell my health care provider before using potassium acid phosphate?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them determine if potassium acid phosphate is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your health care provider if you have any of the following.

Other Medicines and Supplements. Potassium acid phosphate may interact with other medicines and supplements. Before taking potassium acid phosphate, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. It is not known if or how potassium acid phosphate could affect pregnancy or harm a fetus. Tell your health care provider if you are pregnant or plan to become pregnant. Your health care provider will let you know if you should take potassium acid phosphate while you are pregnant or trying to get pregnant.

Breastfeeding. It is not known if potassium acid phosphate passes into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed. Your health care provider will let you know if you should take potassium acid phosphate while breastfeeding.

Interactions

Does potassium acid phosphate interact with foods or drinks?

There are no known interactions between potassium acid phosphate and foods or drinks. Taking potassium acid phosphate with food can help to reduce side effects.

It is unknown if drinking alcohol will affect potassium acid phosphate.

Does potassium acid phosphate interact with other medicines (drug interactions)?

Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. 

In particular, make sure that you discuss if you are using any of the following before taking potassium acid phosphate.

  • Aspirin, which is in many prescription and OTC products, including those for pain, swelling, and fever
  • A salicylate, such as diflunisal or salsalate, which is a medicine used for osteoarthritis or rheumatoid arthritis
  • An antacid containing aluminum, calcium, or magnesium
  • Amiloride (Midamor), eplerenone (Inspra), spironolactone (Aldactone, Carospir), or triamterene (Dyrenium), which are types of diuretics, also called water pills
  • Potassium chloride (Klor-Con and others), which is a medicine used to raise potassium levels
  • A supplement or salt substitute containing potassium

This may not be a complete list of medicines that can interact with potassium acid phosphate. Always check with your health care provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much potassium acid phosphate?

If you or someone else has used too much potassium acid phosphate, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of potassium acid phosphate?

If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and only take the next dose. Do not take double or extra doses.