What is the downside of taking Rennie anti acid for reflux

The short version: Rennie works fast on symptoms but the evidence on its downsides is thin, and the downsides that are documented are about drug interactions, not about what it does to your own digestion over time.

Rennie is a calcium and magnesium carbonate antacid. One in-vitro study shows it raises stomach pH to 5.2 within 40 seconds and keeps it above 3 for about 10 minutes [1]. That is fast symptom relief. But the question is what you trade for it.

The most concrete downside in the evidence is that antacids can block absorption of other drugs. A 1980 study found that magnesium-aluminum hydroxide gel reduced tetracycline absorption by nearly 90% [5]. A 2014 study found that taking an aluminum-magnesium antacid at the same time as the antibiotic nemonoxacin cut its absorption by about 80% [2]. Calcium carbonate alone did not affect nemonoxacin in that study [2], but other evidence shows calcium impairs absorption of ciprofloxacin [4]. If you take Rennie and also take a prescription drug, especially an antibiotic, you may be getting far less of it than you think.

The bigger downside is what the evidence does not study. Rennie neutralizes acid. Acid is there for a reason: it sterilizes food, activates pepsin, and signals the pancreas to release digestive enzymes. A 1982 study in patients with pancreatic insufficiency found that calcium carbonate actually worsened fat malabsorption compared to no antacid [3]. That is a specific population, but it raises the question: if you blunt stomach acid regularly, do you absorb nutrients less well over time? No study in the retrieval answers that for a healthy person taking Rennie occasionally.

There is also the rebound question. When antacid wears off, the stomach can overproduce acid to compensate. The retrieval contains no study on rebound hyperacidity from calcium carbonate specifically. That is a gap, not a finding.

My call: Rennie is fine for occasional use if you are not on other medications. If you take it daily or near other drugs, the evidence does not support that as safe. The studies that would settle the long-term question have not been done. Confidence: moderate for the drug interaction risk, low for everything else.