Mar-Cof CG Side Effects: What to Expect and When to Call Your Doctor

Like all medications that contain an opioid, Mar-Cof CG carries both common side effects and potentially serious risks. Understanding what to expect — and what should prompt an immediate call to your doctor — is essential for using this medication safely.

What Is Mar-Cof CG?

Mar-Cof CG is a prescription combination cough syrup containing codeine phosphate (7.5 mg per teaspoon) and guaifenesin (225 mg per teaspoon). Codeine is an opioid that suppresses the cough reflex in the brain. Guaifenesin is an expectorant that thins mucus. The side effects described below are primarily related to codeine, as guaifenesin has a very favorable tolerability profile.

Boxed Warning: Serious Risks You Need to Know

The FDA requires a boxed warning (the most serious type) for codeine-containing products. Before reviewing common side effects, these critical warnings must be understood:

• Addiction, Abuse, and Misuse: Mar-Cof CG contains codeine, an opioid. Misuse can lead to addiction, overdose, or death — especially in children or people using it without a prescription.
• Life-Threatening Respiratory Depression: Codeine can slow or stop your breathing. The risk is highest in the first 24–72 hours of use and with dose changes. This risk is significantly elevated when combined with alcohol, benzodiazepines, or other CNS depressants.
• Not for Children Under 18: The FDA recommends against use in patients under 18. Some children are CYP2D6 ultra-rapid metabolizers, meaning codeine converts to dangerously high levels of morphine in their bodies. Deaths have been reported.

Common Side Effects of Mar-Cof CG

These side effects are relatively common and often improve as your body adjusts to the medication:

• Drowsiness / Sedation: One of the most common effects of codeine. Do not drive or operate machinery until you know how this medication affects you.
• Dizziness / Lightheadedness: Rise slowly from sitting or lying positions to reduce the risk of dizziness-related falls.
• Nausea / Vomiting: Taking the medication with food or milk may help reduce nausea.
• Constipation: A common opioid side effect. Increased fluid and fiber intake can help. For short courses, this typically resolves when the medication is stopped.
• Facial Flushing: Mild redness or warmth in the face is a known codeine effect.

Serious Side Effects: When to Stop the Medication and Call Your Doctor

Stop taking Mar-Cof CG and contact your doctor or seek emergency care immediately if you experience:

• Slow, shallow, or difficult breathing — this is a sign of respiratory depression and is a medical emergency
• Blue-colored lips or fingernails (cyanosis) — indicates oxygen deprivation; call 911 immediately
• Extreme difficulty staying awake or being woken up — call 911 immediately
• Severe dizziness, confusion, or mental status changes
• Pinpoint pupils
• Signs of allergic reaction: hives, difficulty breathing, swelling of the face, lips, tongue, or throat

When to Call Your Doctor (Non-Emergency)

Also contact your doctor (but don't necessarily call 911) if:

• Your cough has not improved after 7 days of treatment
• Your cough is accompanied by fever, rash, or persistent headache
• Side effects like drowsiness or nausea are significantly impacting daily activities
• You are experiencing severe constipation that does not improve

Overdose Signs: Know What to Watch For

A codeine overdose can be fatal. Call 911 immediately if you or someone you know shows signs of codeine overdose:

• Slow, shallow breathing or stopped breathing
• Severe drowsiness — unable to be woken up
• Muscle weakness, cold and clammy skin
• Pinpoint pupils and fainting

You can also call the Poison Help line at 1-800-222-1222. Naloxone (Narcan) can reverse a codeine/opioid overdose — if you or someone in your household takes opioids, talk to your doctor or pharmacist about keeping naloxone on hand.

Tips for Minimizing Side Effects

• Take only the prescribed dose — do not exceed it
• Avoid alcohol, sleep medications, and other CNS depressants while using this medication
• Use only for the short term — no more than 7 days unless directed by your doctor
• Store out of reach of children and in a secure location

For a full list of drug interactions to discuss with your doctor, see our guide: Mar-Cof CG Drug Interactions.