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Withdrawal symptoms — possibly including headache, irritability, an inability to concentrate, and stomach aches[citation needed] — may appear within 12 to 24 hours after discontinuation of caffeine intake, peak at roughly 48 hours, and usually last from one to five days - representing the time required for the number of adenosine receptors in the brain to revert to "normal" levels, uninfluenced by caffeine consumption. Caffeine causes excess release of stomach acids during ingestion.[citation needed] When in withdrawal the stomach acid levels decrease substantially and can cause some stomach aches in certain people.[citation needed] The aches normally last between 24-48 hours and can be confused with constipation.[citation needed] Analgesics, such as aspirin, can relieve the pain symptoms, as can a small dose of caffeine.[43] Most effective is a combination of both an analgesic and a small amount of caffeine.