East side asked
Can long use or too high of a dosage of metformin cause a rash and itching on the arms and legs? One of my doctors said yes, it was a photosensitivity reaction. I was taking two 500 mg of metformin ER twice a day.
At a glance
- Metformin isn't generally associated with increasing sun sensitivity, but there are a few published case reports describing this reaction.
- Other diabetes drugs however, including glipizide and glyburide, are associated with increasing sensitivity to the sun.
Answer
A number of drugs are known to cause sensitivity to the sun, increasing the likelihood of burns or associated skin reactions (e.g. a rash). These are known as "photosensitivity reactions" and are generally classified as a 'phototoxic' or 'photoallergic' reaction.
A 'photoallergic reaction' is immune-mediated and often happens after continuous dosing of a drug or with ones that are applied topically. They are considered relatively rare.
'Phototoxic reactions' are the most common and occur when the offending drug absorbs UVA light and causes damage in the skin. It typically manifests as an exaggerated sunburn or rash.
Does Metformin Cause Photosensitivity?
Metformin is not typically associated with photosensitivity reactions or increased sensitivity to light. It isn't listed as a possible reaction in the prescribing information for metformin products and is not listed by the FDA as a medication associated with sun sensitivity.
Nevertheless, the prescribing information does list 'rash' as a rare, miscellaneous side effect of metformin, with a reported 1-5% incidence, so the rash you are experiencing may be from the drug.
In addition, there have been rare case reports of metformin causing sun sensitivity.
One study described 3 case reports where the onset of a reaction (photosensitive) was well after initiation with the drug (22 days, 4 months and 4 years for the 3 cases respectively.) However, even though metformin was suspected as the causative agent, it wasn't conclusively shown to be responsible.
Diabetes Drugs Associated With Sun Sensitivity
Although metformin specifically isn't generally associated with sun sensitivity reactions, one commonly used class of anti-diabetic drugs are the sulfonylureas. These drugs include:
If you are taking any of the above agents, they may be responsible for the rash you are experiencing.
Manage Drugs That Cause Photosensitivity Reactions
If you are taking a drug known to cause sun sensitivity, the best form of prevention is to avoid sun exposure or protect yourself with clothing and sunblock. One source states that taking your medication at bedtime, if possible, may decrease the risk.
If you do experience a reaction, discontinuing the drug is often the best course of action but lowering the dose may help reduce your risk of future reactions as well. Topical steroids can help resolve the reactions sooner and relieve itching.
Final Words
Metformin isn't associated with increased sun sensitivity or photosensitivity reactions. However, 'rash' is listed as a rare side effect of the drug and there have been a few case reports of metformin photosensitivity reactions. Be sure to discuss with your doctor the side effects you are experiencing.
References
- Photosensitivity. NEJM
- Drug-induced photosensitivity: culprit drugs, management and prevention. PubMed
- Drug-induced cutaneous photosensitivity: incidence, mechanism, prevention and management. PubMed
- [Photosensitivity induced by metformin: a report of 3 cases]. EuropePMC
- The Sun and Your Medicine. FDA
- Glucophage Prescribing Information. BMS
- Phototoxic and photoallergic cutaneous drug reactions. PubMed
