The short version
- A flare is more pain in the first day or two, then settling. It is a recognised reaction, not a sign the injection failed.
- If you have diabetes, expect a real rise in blood sugar for about three days. In one continuous-monitoring study it averaged roughly 23 mg/dL and nobody needed a treatment change.
- The genuinely serious complication is joint infection, and it is rare — but it looks different from a flare, and the difference is the most useful thing on this page.
- Honest caveat: the safety literature is thinner than the efficacy literature. Most of it is retrospective, and incidence figures vary widely.
Side effects are the part of injections least well covered by the research, which is worth saying at the outset. A 2023 review of intra-articular glucocorticoid safety concluded that most studies report only severe adverse events, that reported incidences vary considerably, and that the data are largely retrospective. So the honest framing here is patterns and mechanisms, with numbers where good numbers exist.
What is a cortisone flare?
An increase in pain in the injected joint, typically starting within hours and lasting a day or two before settling. It is thought to be a reaction to the crystalline steroid preparation itself rather than a sign of anything going wrong, and it is listed among the recognised short-term local complications alongside injection-site pain, skin changes and, rarely, tendon rupture.
Practically it means the first forty-eight hours are not a fair test of whether the injection worked. People sometimes conclude on day one that it failed, when the actual answer arrives around day three to seven.
What generally helps in the meantime: ice over the area, keeping the joint relatively quiet for a day or two rather than testing it, and whatever you would normally take for pain — check with your own clinician or pharmacist if you are unsure what is appropriate alongside your other medications. What does not help is repeatedly stress-testing the joint to see whether it still hurts.
What I tell people beforehand
That it may hurt more tonight and tomorrow, that ice and keeping the joint quiet help, and that this is expected rather than a complication. Saying it in advance changes how the same experience is interpreted — an anticipated flare is inconvenient, an unexpected one is frightening.
That is a communication habit built over thousands of these rather than a trial finding. But it is the single change that has most reduced worried phone calls the following day.
How much does it raise blood sugar?
This is the best-quantified side effect, and the numbers are reassuring without being trivial.
A study using continuous glucose monitoring in 25 patients with type 2 diabetes not on insulin, following a shoulder injection, tracked glucose for three days before and eleven days after.
| Measure | What happened |
|---|---|
| Mean glucose | Rose from 136 to 159 mg/dL over the first three days, then returned to baseline. |
| Time above 250 mg/dL | Rose from 4.3% before to 9.5% on day one, then 7% on day two and 3.8% on day three. |
| Readings above 350 mg/dL | New in four of twenty-five patients, and short-lived in all four. |
| Treatment changes needed | None. No patient required additional medication or insulin. |
So: a real and sometimes substantial rise, concentrated in the first two to three days, that in this group did not require intervention. The practical implication is to check more often than usual for a few days rather than to avoid the injection — and to tell whoever manages your diabetes that you are having one.
The other side effects
Grouped by how often they actually turn up:
- Common and self-limiting — injection-site soreness, facial flushing for a day or two, a temporary lift or dip in mood, increased appetite.
- Uncommon and usually cosmetic — thinning or dimpling of the fat under the skin at the injection site, and loss of skin pigment. More noticeable on darker skin and in superficial injections; often partial and slow to recover, sometimes permanent.
- Rare but important — tendon rupture where steroid is placed close to a tendon, and bleeding in people on anticoagulants.
- Rare and serious — septic arthritis. This is the complication that drives the sterile technique and the one everything else is measured against.
There is also a systemic dimension that gets underestimated. A review of systemic absorption found that locally injected glucocorticoid can produce measurable systemic effects lasting weeks, with wide individual variability, and that cumulative exposure should be counted across all injections rather than treated joint by joint. That is a reason to know your running total — not a reason to avoid a single injection.
When should you actually call?
A flare and an infection can both mean "the joint hurts more." They separate on timing and direction.
Seek evaluation the same day if
The pain is getting worse after day two rather than better. A flare peaks early and settles. Pain that escalates on day three or four is going the wrong way.
There is fever, chills, or spreading redness and heat around the joint.
The joint becomes hot, very swollen, and difficult to move at all — rather than sore but usable.
Septic arthritis is uncommon, but it is a surgical emergency and delay causes joint damage. Call the clinician who performed the injection first — and if you cannot reach someone promptly, go to urgent care or an emergency department rather than waiting for a callback.
Most post-injection pain is a flare and turns out to be nothing. Nobody minds being called or seen about one; the reverse is what causes harm.
Also worth a call, though not urgently: blood sugar that stays high beyond about four days, or a dimple or pale patch developing at the injection site — mostly so it is documented and factored into any future injection.
Sources
- Safran O, Fraind-Maya G, Kandel L, et al. The effect of steroid injection into the shoulder on glycemia in patients with type 2 diabetes. JSES International 2022;6(5):843–848. Continuous glucose monitoring, 25 patients. Retrieved via PubMed.
- Stout A, Friedly J, Standaert CJ. Systemic absorption and side effects of locally injected glucocorticoids. PM&R 2019;11(4):409–419. Retrieved via PubMed.
- Duarte-Monteiro AM, Dourado E, Fonseca JE, Saraiva F. Safety of intra-articular glucocorticoid injections — state of the art. ARP Rheumatology 2023;2(1):64–73. Review noting that most safety data are retrospective and reported incidences vary. Retrieved via PubMed.