Withdrawal symptoms can come back within days, weeks, or even months after detox, depending on the substance and your overall recovery plan. For alcohol, acute withdrawal can start within hours and usually peaks in the first 24–72 hours, while some post-acute symptoms like anxiety, sleep issues, and cravings can linger much longer. For opioids and benzodiazepines, symptoms may also return later as part of post-acute withdrawal, so ongoing support after detox is really important.