Phrenilin Side Effects

Generic name: acetaminophen / butalbital

Medically reviewed by Drugs.com. Last updated on May 1, 2025.

Note: This document provides detailed information about Phrenilin Side Effects associated with acetaminophen / butalbital. Some dosage forms listed on this page may not apply specifically to the brand name Phrenilin.

Applies to acetaminophen / butalbital: oral capsule, oral tablet.

Precautions

It is very important that your doctor check your progress while you are using this medicine, especially within the first 24 to 72 hours of treatment. This will allow your doctor to see if the medicine is working properly and to decide if you should continue to take it. Blood tests may be needed to check for unwanted effects.

This medicine may cause serious skin reactions (eg, acute generalized exanthematous pustulosis, Stevens-Johnson syndrome, toxic epidermal necrolysis). Check with your doctor right away if you have blistering, peeling, or loosening of the skin, chills, cough, diarrhea, itching, joint or muscle pain, red irritated eyes, red skin lesions, often with a purple center, sore throat, sores, ulcers, or white spots in the mouth or on the lips, or unusual tiredness or weakness.

This medicine may cause serious allergic reactions, including anaphylaxis, which can be life-threatening and require immediate medical attention. Call your doctor right away if you have chest tightness, cough, dizziness, fast heartbeat, hives, rash, itching, puffiness or swelling of the eyelids, or around the eyes, face, lips, or tongue, trouble breathing or swallowing, or unusual tiredness or weakness while you are using this medicine.

This medicine may make you dizzy, drowsy, confused, or disoriented. Do not drive or do anything else that could be dangerous until you know how this medicine affects you.

Do not suddenly Stop taking acetaminophen / butalbital without first checking with your doctor. Your doctor may want you to gradually reduce the amount you are using before stopping completely. This may help prevent a possible worsening of your condition and reduce the possibility of withdrawal symptoms such as confusion as to time, place, or person, hallucinations, holding false beliefs that cannot be changed by fact, seizures, or unusual excitement, nervousness, or restlessness.

This medicine may be habit-forming. If you feel that the medicine is not working as well, do not use more than your prescribed dose. Call your doctor for instructions.

Before you have any medical tests, tell the medical doctor in charge that you are taking this medicine. The results of some tests may be affected by this medicine.

This medicine will add to the effects of alcohol and other CNS depressants (medicines that can make you drowsy or less alert). Some examples of CNS depressants are antihistamines or medicine for allergies or colds, sedatives, tranquilizers, or sleeping medicine, other prescription pain medicine or narcotics, medicine for seizures or barbiturates, muscle relaxants, or anesthetics, including some dental anesthetics. Also, there may be a greater risk of liver damage if you drink 3 or more alcoholic beverages while you are taking acetaminophen. Do not drink alcoholic beverages, and check with your doctor before taking any of these medicines while you are using this medicine.

If you think you or someone else may have taken an overdose of this medicine, get emergency help at once. Signs of an overdose include: black, tarry stools, bloody or cloudy urine, blurred vision, change in consciousness, chills, confusion, dark urine, difficult or painful urination, dizziness, faintness, or lightheadedness when getting up suddenly from a lying or sitting position, fever, headache, irregular, fast or slow, or shallow breathing, itching, light-colored stools, loss of appetite, loss of consciousness, nausea, pale or blue lips, fingernails, or skin, sleepiness, stomach pain, sudden decrease in amount of urine, sweating, trouble breathing, unpleasant breath odor, unusual tiredness or weakness, vomiting of blood, or yellow eyes or skin.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

Serious side effects of Phrenilin

Along with its needed effects, acetaminophen / butalbital may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking acetaminophen / butalbital:

Get emergency help immediately if any of the following symptoms of overdose occur while taking acetaminophen / butalbital:

Symptoms of overdose

• black, tarry stools
• bloody or cloudy urine
• blurred vision
• change in consciousness
• chills
• confusion
• dark urine
• difficult or painful urination
• dizziness, faintness, or lightheadedness when getting up suddenly from a lying or sitting position
• fever
• headache
• irregular, fast or slow, or shallow breathing
• itching
• light-colored stools
• loss of appetite
• loss of consciousness
• nausea
• pale or blue lips, fingernails, or skin
• sleepiness
• stomach pain
• sudden decrease in amount of urine
• sweating
• trouble breathing
• unpleasant breath odor
• unusual tiredness or weakness
• vomiting of blood
• yellow eyes or skin

Other side effects of Phrenilin

Some side effects of acetaminophen / butalbital may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

For healthcare professionals

Applies to acetaminophen / butalbital: oral capsule, oral tablet.

General adverse events

The most frequently reported adverse events have included drowsiness, lightheadedness, dizziness, sedation, shortness of breath, nausea, vomiting, abdominal pain, and intoxicated feeling.^[Ref]

Hepatic

Acetaminophen:

• Frequency not reported: Liver failure^[Ref]

Acute liver failure has been reported with acetaminophen use; many case have been associated with use at doses in excess of 4 g/day, and often involving more than 1 acetaminophen-containing product.^[Ref]

Hypersensitivity

Butalbital-Acetaminophen:

• Uncommon (0.1% to 1%): Allergic reactions
• Postmarketing reports: Anaphylaxis

Acetaminophen:

• Postmarketing reports: Anaphylaxis^[Ref]

Dermatologic

Butalbital-Acetaminophen:

• Uncommon (0.1% to 1%): Hyperhidrosis, pruritus
• Frequency not reported: Toxic epidermal necrolysis, erythema multiforme

Acetaminophen:

• Rare (less than 0.1%): Serious skin reactions such as acute generalized exanthematous pustulosis, Stevens-Johnson syndrome, and toxic epidermal necrolysis
• Frequency not reported: Rash^[Ref]

Several cases of dermatologic reactions including toxic epidermal necrolysis and erythema multiforme have been reported.^[Ref]

Psychiatric

• Uncommon (0.1% to 1%): Agitation, euphoria^[Ref]

Nervous system

• Common (1% to 10%): Drowsiness, lightheadedness, dizziness, sedation
• Uncommon (0.1% to 1%): Headache, shaky feeling, tingly, fainting, numbness, seizures, mental confusion, excitement or depression due to intolerance^[Ref]

Respiratory

• Common (1% to 10%): Shortness of breath
• Uncommon (0.1% to 1%): Nasal congestion^[Ref]

Gastrointestinal

• Common (1% to 10%): Nausea, vomiting, abdominal pain
• Uncommon (0.1% to 1%): Dry mouth, difficulty swallowing, heartburn, flatulence, constipation^[Ref]

Hematologic

Acetaminophen:

• Frequency not reported: Thrombocytopenia, agranulocytosis^[Ref]

Cardiovascular

• Uncommon (0.1% to 1%): Tachycardia^[Ref]

Musculoskeletal

• Uncommon (0.1% to 1%): Leg pain, muscle fatigue^[Ref]

Other

• Common (1% to 10%): Intoxicated feeling
• Uncommon (0.1% to 1%): Fatigue, high energy, heavy eyelids, sluggishness, hot spells, fever, earache, tinnitus^[Ref]

Renal

• Uncommon (0.1% to 1%): Diuresis,^[Ref]

See also:

Further information

Phrenilin side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Note: Medication side effects may be underreported. If you are experiencing side effects that are not listed, submit a report to the FDA by following this guide.

Medical Disclaimer