Lettuce that's suddenly bitter and sending up a tall central stalk is bolting, the plant switching from leaf production into flowering and seed production, and it's one of the most reliable disappointments in a summer vegetable garden. The bitterness and the bolting aren't two separate problems, they're the same one: as a lettuce bolts, it produces more of the milky latex sap that gives all lettuce its faint natural bitterness, and the redirected energy toward flowering concentrates it fast.
Heat is the main trigger. Lettuce is a cool-season crop, and once temperatures run consistently warm, particularly a run of hot days rather than one isolated warm afternoon, it reads that as a signal to rush toward seed before conditions get worse. Longer daylight hours play a role too in many varieties, which is part of why lettuce that sails through a mild spring so often bolts within a couple of weeks once summer properly arrives, even without a dramatic heatwave.
Once a lettuce has visibly started sending up its central flower stalk, there's no reversing it. The leaves already on the plant will only get more bitter from that point, not less, so the useful move is to harvest what's there rather than waiting for it to improve. Pulling the plant and resowing in a cooler spot, or waiting for temperatures to ease, is more productive than trying to nurse a bolted plant along.
Once a lettuce has visibly started sending up its central flower stalk, there's no reversing it.
A few things genuinely slow bolting down rather than just delaying the inevitable. Consistent moisture matters more than most gardeners expect: a lettuce under drought stress bolts noticeably faster than one kept evenly watered, since the stress itself is part of what triggers the response. Partial shade through the hottest part of the day helps in warmer regions, and choosing a slow-bolt or heat-tolerant variety for summer sowings buys real extra weeks over a standard variety planted at the same time.
Sowing little and often, rather than one large batch, is the most reliable long-term fix. A continuous run of small sowings every couple of weeks means some lettuce is always at a good eating stage while an earlier batch bolts, instead of losing an entire planting to heat all at once.