Blue Capsule Pills: Identifying RX693, C 39, and M 42

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Blue Capsule Pills: Identifying RX693, C 39, and M 42

By Your Health Magazine Health Information Team

You find a loose blue capsule in a travel bag or medicine cabinet. The printing appears to say “RX693,” “C 39,” or “M 42,” but you are not certain. Is it an antibiotic, a pain medication, or something else? Color alone cannot answer that question. The imprint, capsule shape, packaging, and prescription label must all match before a medication can be considered reliably identified.

In current U.S. prescription labeling, the blue capsule pill RX693, the blue capsule M 42, and the blue capsule C 40 are identified as 300-milligram clindamycin hydrochloride capsules. The C 39 pill is also clindamycin, but it contains 150 milligrams and typically has a light-blue cap with a light-green body. These products contain the same antibiotic in different strengths or manufacturer-specific designs.

Quick Identification Guide

• RX693: A turquoise or blue opaque capsule printed “RX693,” commonly on both halves. It contains clindamycin hydrochloride equivalent to 300 milligrams of clindamycin.
• C 39: A capsule with “C” on the light-blue cap and “39” on the light-green body. It contains 150 milligrams of clindamycin.
• C 40: A light-blue capsule with “C” on one half and “40” on the other. It contains 300 milligrams of clindamycin.
• M 42: A light-blue capsule printed with “M” on the cap and “42” on the body. It contains 300 milligrams of clindamycin.

The spaces in an imprint may be reported differently. For example, blue capsule M42 and M 42 generally refer to the same imprint, while pill C 39 may appear in databases as “C;39.” An unclear, worn, or partial imprint should never be guessed.

What Is Clindamycin?

Clindamycin is a prescription antibiotic used to treat certain bacterial infections. Depending on the clinical situation, a healthcare professional may prescribe it for infections involving the skin, soft tissues, lungs, bones, joints, teeth, or internal organs. It slows or stops the growth of susceptible bacteria.

Clindamycin does not treat colds, influenza, or other viral illnesses. Taking an antibiotic when it is not needed can cause side effects and contribute to antibiotic resistance. Never take a capsule simply because its appearance resembles medication that was previously prescribed to you.

Readers can review general medication safety and patient drug information through the MedlinePlus drug information library.

Why Several Capsules Contain the Same Medicine

Generic medications may be produced or repackaged by different companies. Each manufacturer uses an identifying imprint and may select a different shade, capsule size, or color combination. That is why RX693, C 40, and M 42 can all represent 300-milligram clindamycin even though their markings do not match.

This also explains why a refill may look different from an earlier prescription. A pharmacy may have changed suppliers while dispensing the same active ingredient and strength. Even so, contact the pharmacist before taking a refill that looks unexpectedly different. The pharmacist can compare the imprint, manufacturer, prescription record, and original container.

Important Safety Warning About Diarrhea

Clindamycin carries a significant risk of antibiotic-associated diarrhea and inflammation of the colon. Antibiotics can alter normal intestinal bacteria, allowing harmful bacteria such as Clostridioides difficile, often called C. diff, to multiply. The resulting illness can range from mild diarrhea to severe colitis.

Watery or bloody stools, abdominal cramps, and fever can develop while taking clindamycin or several months after treatment has ended. Do not automatically treat significant diarrhea with an over-the-counter anti-diarrheal medicine. Contact a healthcare professional promptly because suppressing bowel movements may be inappropriate in some infections.

Other Possible Side Effects

Some people taking clindamycin experience nausea, vomiting, an unpleasant or metallic taste, heartburn, or vaginal irritation and discharge. A capsule can also irritate the esophagus, particularly if it becomes lodged in the throat. Follow the prescription directions, swallow the capsule whole with a full glass of water, and remain upright afterward.

More serious reactions may include hives, swelling of the face or throat, breathing difficulty, severe skin blistering or peeling, yellowing of the skin or eyes, reduced urination, or painful swallowing. These symptoms require prompt medical attention.

Tell the prescriber and pharmacist about allergies, previous colitis or other intestinal disease, liver or kidney problems, pregnancy, breastfeeding, and every prescription drug, nonprescription medicine, vitamin, or supplement you use. This information helps them evaluate whether clindamycin is appropriate and check for interactions.

What to Do With an Unknown Blue Capsule

Do not swallow, share, crush, or taste a capsule that is loose or cannot be matched confidently to its labeled container. Instead:

• Read the complete imprint under bright light and check both sides.
• Note whether the capsule is entirely blue or blue and green.
• Compare it with the pharmacy label and remaining pills in the original bottle.
• Ask a pharmacist to identify it before anyone takes it.
• Keep it away from children, pets, and anyone who might take it accidentally.
• Use a pharmacy or community medication take-back option if it is unneeded or cannot be identified.

A visual resemblance does not prove a pill’s contents. Counterfeit, damaged, imported, or improperly stored products may not match official descriptions. If you are trying to distinguish an antibiotic from an opioid pain medicine, see this guide to what a Dilaudid pill looks like and how to identify it. Clindamycin capsules marked RX693, C 39, C 40, or M 42 are not Dilaudid.

When to Seek Care

Contact the prescribing clinician or a pharmacist if the capsule does not match the label, the imprint is unreadable, you miss medication, your infection is not improving, or you develop persistent side effects. Seek urgent medical evaluation for watery or bloody diarrhea, severe abdominal pain, fever, extensive rash, blistering skin, yellow skin or eyes, or difficulty swallowing.

Call 911 for trouble breathing, facial or throat swelling, collapse, seizure, or inability to awaken. If an unidentified capsule was swallowed or too much medication may have been taken, contact Poison Control at 1-800-222-1222 promptly, even if no symptoms are present.

The Bottom Line

The blue capsule pill RX693 is generally clindamycin 300 milligrams. Blue capsules marked C 40 or M 42 also contain 300 milligrams of clindamycin, while the blue-and-green C 39 pill contains 150 milligrams. These descriptions are useful screening clues, not substitutes for the original prescription container or professional verification. When any detail is uncertain, set the capsule aside and ask a pharmacist to confirm it.